Only install completions for the "drush" command

This is a potential fix for issue #915. I've tried to document the procedure everywhere that aliases/bash-completion are already mentioned in the docs.

It's not _quite_ that simple. The `_drush_completion()` function won't be in scope in your ~/.bashrc, but if you're willing to source the completions file first it works. For example, when drush (and its completion file) is installed globally:

```bash
source /usr/share/bash-completion/completions/drush
complete -o bashdefault -o default -o nospace -F _drush_completion dr
```