make prompts a little more explicit

i'm using ghauth in https://www.npmjs.org/package/collaborator and it's a little confusing because you do

```
$ cd myrepo
$ collaborator rvagg
$ GitHub Username: maxogden
$ GitHub Password: *********
```

this changes it to: 
```
$ cd myrepo
$ collaborator rvagg
$ Your GitHub Username: maxogden
$ Your GitHub Password: *********
```

which helps clarify that they should enter e.g. `maxogden` instead of `rvagg`