Improved autocompletion for bash

Now it works this way:

````
$ vagrant [TAB][TAB]
box         destroy     halt        init        package
plugin      provision   reload
resume      ssh         ssh-config  status      suspend     up 

$ vagrant b[TAB]
$ vagrant box

$ vagrant box [TAB][TAB]
add        list       remove     repackage

$ vagrant box a[TAB]
$ vagrant box add

$ vagrant box add -[TAB][TAB]
-f          --force     -h          --help      --insecure  --provider

$ vagrant box add --pr[TAB]
$ vagrant box add --provider
````