Fix alt key behaviour as per issue 75
Fix alt key behaviour as per issue 75

When alt key is pressed during key event, change keyCode to read
<a-{keyCode}>

Robinson10240/vimium,smblott-github/vimium,borgified/vimium,poacher2k/vimium,csmalin/vimium,smblott-github/vimium,hhsue/vimium,cyrixhero/vimium,jb55/vimium-plus,mrmr1993/vimium,douglas-larocca/vimium,hhsue/vimium,Aleroniponi/vimium,elderbas/vimium,smblott-github/vimium,cyrixhero/vimium,tuchangwei/vimium,Aleroniponi/vimium,gdh1995/vimium,Lw-Cui/vimium,crazypenguincode/vimium,crazypenguincode/vimium,dimatter/vimium,VPashkov/vimium,elegantwww/vimium,jb55/vimium-plus,csmalin/vimium,poacher2k/vimium,willsALMANJ/vimium,zhangmin510/vimium,gdh1995/vimium,PrestanceDesign/vimium,decaffeinate-examples/vimium,shaung/vimium,Robinson10240/vimium,decaffeinate-examples/vimium,elderbas/vimium,tuchangwei/vimium,PickRelated/vimium,Lw-Cui/vimium,justalittlenoob/vimium,cyrixhero/vimium,mrmr1993/vimium,csmalin/vimium,willsALMANJ/vimium,wildeyes/vimium,PickRelated/vimium,philc/vimium,Lw-Cui/vimium,decaffeinate-examples/vimium,poacher2k/vimium,Aleroniponi/vimium,PickRelated/vimium,douglas-larocca/vimium,bkudria/vimium,borgified/vimium,dimatter/vimium,elegantwww/vimium,zhangmin510/vimium,Robinson10240/vimium,llimllib/vimium,justalittlenoob/vimium,elderbas/vimium,wildeyes/vimium,PrestanceDesign/vimium,bkudria/vimium,tuchangwei/vimium,dimatter/vimium,smblott-github/vimium-smblott,hhsue/vimium,PrestanceDesign/vimium,mrmr1993/vimium,borgified/vimium,VPashkov/vimium,elegantwww/vimium,philc/vimium,justalittlenoob/vimium,zhangmin510/vimium,philc/vimium,smblott-github/vimium-smblott,douglas-larocca/vimium,bkudria/vimium,wildeyes/vimium,willsALMANJ/vimium,gdh1995/vimium,VPashkov/vimium