main: enable basic subcommand autocomplete
main: enable basic subcommand autocomplete

The CLI package has automatic support for shell autocomplete (bash and
zsh, at time of writing) for subcommands, so all we need to do here is
just opt into it.

Users can install this into their shells by running:
    terraform -install-autocomplete

ColinHebert/terraform,lusis/terraform,yanndegat/terraform,apparentlymart/terraform,donaldguy/terraform,VladRassokhin/terraform,ewbankkit/terraform,iansheridan/terraform,donaldguy/terraform,svanharmelen/terraform,asedge/terraform,ddegoede/terraform,ColinHebert/terraform,tommynsong/terraform,catsby/terraform,svanharmelen/terraform,gechr/terraform,rkhozinov/terraform,donaldguy/terraform,drebes/terraform,drebes/terraform,mevansam/terraform,rkhozinov/terraform,VladRassokhin/terraform,VladRassokhin/terraform,asedge/terraform,kl4w/terraform,gechr/terraform,kwilczynski/terraform,catsby/terraform,fatmcgav/terraform,hashicorp/terraform,grasshoppergroup/terraform,apparentlymart/terraform,ddegoede/terraform,huydinhle/terraform,kjmkznr/terraform,heimweh/terraform,yanndegat/terraform,ddegoede/terraform,kwilczynski/terraform,hashicorp/terraform,youhong316/terraform,ddegoede/terraform,brandontosch/terraform,huydinhle/terraform,tommynsong/terraform,nlamirault/terraform,iljaskevic/terraform,hartzell/terraform,nlamirault/terraform,grasshoppergroup/terraform,youhong316/terraform,luis-silva/terraform,donaldguy/terraform,ColinHebert/terraform,VladRassokhin/terraform,kl4w/terraform,mattray/terraform,antonbabenko/terraform,lusis/terraform,youhong316/terraform,antonbabenko/terraform,justnom/terraform,vancluever/terraform,pryorda/terraform,ewbankkit/terraform,iansheridan/terraform,HotelsDotCom/terraform,iansheridan/terraform,yanndegat/terraform,grasshoppergroup/terraform,tpounds/terraform,antonbabenko/terraform,brandontosch/terraform,justnom/terraform,tpounds/terraform,pradeepbhadani/terraform,svanharmelen/terraform,heimweh/terraform,rkhozinov/terraform,kl4w/terraform,chrisjharding/terraform,kjmkznr/terraform,chrisjharding/terraform,rnaveiras/terraform,asedge/terraform,youhong316/terraform,fatmcgav/terraform,mattray/terraform,catsby/terraform,hartzell/terraform,heimweh/terraform,hooklift/terraform,HotelsDotCom/terraform,mattray/terraform,huydinhle/terraform,paultyng/terraform,pradeepbhadani/terraform,iljaskevic/terraform,pdecat/terraform,chrisjharding/terraform,drebes/terraform,kwilczynski/terraform,paultyng/terraform,brandontosch/terraform,pdecat/terraform,vancluever/terraform,rnaveiras/terraform,apparentlymart/terraform,vancluever/terraform,brandontosch/terraform,kl4w/terraform,asedge/terraform,yanndegat/terraform,grasshoppergroup/terraform,pryorda/terraform,svanharmelen/terraform,tommynsong/terraform,nlamirault/terraform,luis-silva/terraform,justnom/terraform,ColinHebert/terraform,hooklift/terraform,HotelsDotCom/terraform,tpounds/terraform,apparentlymart/terraform,mevansam/terraform,fatmcgav/terraform,drebes/terraform,gechr/terraform,catsby/terraform,kjmkznr/terraform,hashicorp/terraform,rkhozinov/terraform,tpounds/terraform,heimweh/terraform,vancluever/terraform,hartzell/terraform,kwilczynski/terraform,apparentlymart/terraform,luis-silva/terraform,rnaveiras/terraform,iansheridan/terraform,heimweh/terraform,hartzell/terraform,brandontosch/terraform,hooklift/terraform,kjmkznr/terraform,mevansam/terraform,ewbankkit/terraform,lusis/terraform,pradeepbhadani/terraform,mattray/terraform,huydinhle/terraform,antonbabenko/terraform,pdecat/terraform,antonbabenko/terraform,justnom/terraform,iljaskevic/terraform,ddegoede/terraform,pryorda/terraform,HotelsDotCom/terraform,iljaskevic/terraform,chrisjharding/terraform,pdecat/terraform,HotelsDotCom/terraform,paultyng/terraform,paultyng/terraform,luis-silva/terraform,mevansam/terraform,chrisjharding/terraform,ewbankkit/terraform,lusis/terraform,fatmcgav/terraform,hooklift/terraform