Shell completion

I recently created `grunt-init-completion` to add shell completion for `grunt-init`. See https://npmjs.org/package/grunt-init-completion.

``` sh
npm install --global grunt-init-completion
grunt-init-completion completion --save
```

I decided to do this with an external module because shell completion is confusing enough as is and I thought this would be simpler, but I am willing to have it added to the core.