Prompt for auth token when no args provided
Prompt for auth token when no args provided

This makes `vault auth` work as documented:

> If no -method is specified, then the token is expected. If it is not
> given on the command-line, it will be asked via user input. If the
> token is "-", it will be read from stdin.

hashicorp/vault,ModusCreateOrg/vault,DmitryOlshansky/vault,pavankumar2203/vault,drampelt/vault,hashicorp/vault,tyamell/vault,nicr9/vault,curtiszimmerman/vault,lemondrank/vault,grange74/vault,justincampbell/vault,jklein/vault,theonewolf/vault,Zenithar/vault,nicr9/vault,DmitryOlshansky/vault,rickfast/vault,Banno/vault,mgk/vault,doubledutch/vault,quixoten/vault,dennisme/vault,theonewolf/vault,doubledutch/vault,DmitryOlshansky/vault,careyblack/vault,yanzay/vault,eladapps/vault,annanay25/vault,pradeepchhetri/vault,chiefy/vault,ryon/vault,mymercurialsky/vault,hobbeswalsh/vault,zendesk/vault,justincampbell/vault,Zagorakiss/vault,captainill/vault,geckoboard/vault,jklein/vault,skippy/vault,nbrownus/vault,ekristen/vault,dragonndev/vault,sofuture/vault,captainill/vault,jklein/vault,doubledutch/vault,dennisme/vault,grange74/vault,vicki-c/vault,jgcallero/vault,Zenithar/vault,careyblack/vault,zBMNForks/vault,hashbrowncipher/vault,MarkVLK/vault,doodles526/vault,naunga/vault,hartsock/vault,kgutwin/vault,tyamell/vault,geckoboard/vault,jmhardison/vault,ipoval/vault,drampelt/vault,memory/vault,christophermanning/vault,mfischer-zd/vault,hashbrowncipher/vault,grunzwei/vault,pavankumar2203/vault,bkw/vault,kingland/vault,Caiyeon/vault,dennisme/vault,kingland/vault,pakfur/vault,jszwedko/vault,jefferai/vault,hashicorp/vault,ipoval/vault,nicr9/vault,pradeepchhetri/vault,Aloomaio/vault,hashicorp/vault,chiefy/vault,grunzwei/vault,rnaveiras/vault,jklein/vault,ateoto/vault,MonsantoCo/vault,ekristen/vault,geckoboard/vault,tallpauley/vault,databus23/vault,kingland/vault,rickfast/vault,drampelt/vault,careyblack/vault,andrewstuart/vault,msample/vault,jefferai/vault,dragonndev/vault,ateoto/vault,dennisme/vault,joelthompson/vault,skippy/vault,bluecmd/vault,rickfast/vault,broadinstitute/vault,kingland/vault,kenbreeman/vault,hashbrowncipher/vault,Zagorakiss/vault,sepiroth887/vault,Zagorakiss/vault,grunzwei/vault,ipoval/vault,doubledutch/vault,databus23/vault,justincampbell/vault,doodles526/vault,Aloomaio/vault,Banno/vault,pavankumar2203/vault,ekristen/vault,careyblack/vault,chris-west-travelex/vault,krmcbride/vault,tyamell/vault,Aloomaio/vault,MarkVLK/vault,geckoboard/vault,michaelansel/vault,mgk/vault,krmcbride/vault,lassizci/vault,bluecmd/vault,ccpgames/vault,mfischer-zd/vault,sepiroth887/vault,jgcallero/vault,ModusCreateOrg/vault,vicki-c/vault,ateoto/vault,chiefy/vault,ccpgames/vault,wxdublin/vault,ipoval/vault,rickfast/vault,danielsomerfield/vault,doubledutch/vault,jmhardison/vault,bluecmd/vault,annanay25/vault,codeinthehole/vault,Aloomaio/vault,codeinthehole/vault,zBMNForks/vault,nicr9/vault,grunzwei/vault,yanzay/vault,jefferai/vault,Zenithar/vault,curtiszimmerman/vault,yanzay/vault,chris-west-travelex/vault,DmitryOlshansky/vault,rnaveiras/vault,nbrownus/vault,yanzay/vault,andrewstuart/vault,zendesk/vault,nickbourner/vault,quixoten/vault,bgirardeau/vault,nickbourner/vault,wjam/vault,hobbeswalsh/vault,dragonndev/vault,Klaudit/vault,hashicorp/vault,ekristen/vault,andrewstuart/vault,krmcbride/vault,andreaturli/vault,justincampbell/vault,christophermanning/vault,jefferai/vault,careyblack/vault,jam-pot/vault,rnaveiras/vault,ekristen/vault,Klaudit/vault,ModusCreateOrg/vault,broadinstitute/vault,nicr9/vault,kgutwin/vault,hashbrowncipher/vault,MonsantoCo/vault,quixoten/vault,broadinstitute/vault,kingland/vault,drampelt/vault,christophermanning/vault,krmcbride/vault,wjam/vault,cimarron-pistoncloud/vault,MarkVLK/vault,codeinthehole/vault,grange74/vault,mfischer-zd/vault,andreaturli/vault,mhurne/vault,sepiroth887/vault,mtougeron/vault,kenbreeman/vault,jmhardison/vault,hartsock/vault,yanzay/vault,eladapps/vault,bgirardeau/vault,Caiyeon/vault,sofuture/vault,gpndata/vault,msample/vault,curtiszimmerman/vault,kgutwin/vault,sofuture/vault,hartsock/vault,bluecmd/vault,rnaveiras/vault,MarkVLK/vault,databus23/vault,zendesk/vault,captainill/vault,dennisme/vault,nickbourner/vault,gpndata/vault,ipoval/vault,eladapps/vault,bkw/vault,lirao/vault,mhurne/vault,tyamell/vault,broadinstitute/vault,Zagorakiss/vault,Zenithar/vault,zBMNForks/vault,captainill/vault,joelthompson/vault,nbrownus/vault,chris-west-travelex/vault,peteryhwong/vault,theonewolf/vault,curtiszimmerman/vault,sofuture/vault,bkw/vault,kenbreeman/vault,mgk/vault,zBMNForks/vault,mtougeron/vault,zBMNForks/vault,vicki-c/vault,ccpgames/vault,christophermanning/vault,LolalyLuo/vault,lirao/vault,vicki-c/vault,nickbourner/vault,chrishoffman/vault,bluecmd/vault,hartsock/vault,captainill/vault,theonewolf/vault,bgirardeau/vault,peteryhwong/vault,chiefy/vault,gpndata/vault,msample/vault,jefferai/vault,grange74/vault,jszwedko/vault,databus23/vault,andrewstuart/vault,bkw/vault,hobbeswalsh/vault,mgk/vault,wxdublin/vault,andreaturli/vault,joelthompson/vault,Zagorakiss/vault,hartsock/vault,justincampbell/vault,ccpgames/vault,bgirardeau/vault,rickfast/vault,hashicorp/vault,pradeepchhetri/vault,tyamell/vault,gpndata/vault,mtougeron/vault,theonewolf/vault,jszwedko/vault,zendesk/vault,jam-pot/vault,dragonndev/vault,pakfur/vault,eladapps/vault,dragonndev/vault,chrishoffman/vault,grunzwei/vault,Klaudit/vault,DmitryOlshansky/vault,christophermanning/vault,annanay25/vault,sepiroth887/vault,joelthompson/vault,zendesk/vault,lassizci/vault,andrewstuart/vault,nickbourner/vault,jklein/vault,LolalyLuo/vault,sofuture/vault,lassizci/vault,lassizci/vault,bgirardeau/vault,mfischer-zd/vault,pradeepchhetri/vault,pradeepchhetri/vault,ateoto/vault,nbrownus/vault,drampelt/vault,quixoten/vault,msample/vault,martin-ducar-gd/vault,kenbreeman/vault,MarkVLK/vault,rnaveiras/vault,mtougeron/vault,pakfur/vault,kenbreeman/vault,memory/vault,msample/vault,jszwedko/vault,cimarron-pistoncloud/vault,ateoto/vault,eladapps/vault,sepiroth887/vault,broadinstitute/vault,martin-ducar-gd/vault,jmhardison/vault,hashbrowncipher/vault,wxdublin/vault,vicki-c/vault,mymercurialsky/vault,annanay25/vault,ModusCreateOrg/vault,naunga/vault,deverton/hashicorp-vault,codeinthehole/vault,krmcbride/vault,hobbeswalsh/vault,lemondrank/vault,andreaturli/vault,wxdublin/vault,Klaudit/vault,pavankumar2203/vault,lassizci/vault,michaelansel/vault,pakfur/vault,chiefy/vault,Klaudit/vault,mfischer-zd/vault,jszwedko/vault,grange74/vault,hobbeswalsh/vault,kgutwin/vault,nbrownus/vault,codeinthehole/vault,deverton/hashicorp-vault,kgutwin/vault,jmhardison/vault,ModusCreateOrg/vault,mtougeron/vault,annanay25/vault,tallpauley/vault,ryon/vault,wxdublin/vault,Zenithar/vault,chris-west-travelex/vault,geckoboard/vault,andreaturli/vault,pakfur/vault,ccpgames/vault,pavankumar2203/vault,chris-west-travelex/vault,danielsomerfield/vault,databus23/vault,bkw/vault,curtiszimmerman/vault,gpndata/vault