Allow specifying a Drush alias group name (and other minor drush alias features)

This replaces `fix-aliases` with a `drush-aliases` command, usage:
```
platform project:drush-aliases [-r|--recreate] [-g|--group[="..."]] [-p|--pipe]
```

This allows you to specify a new group for all the project's Drush aliases, e.g. `@mysite` instead of `@s0mel0ngPl4tf0rmpr0ject1d`.

Or, if you want to keep the obscure group name, you can use the pipe feature to run commands like ``drush @`platform drush-aliases -p`._local status`` inside any project.

This PR also sets a `'uri'` key on Drush aliases, useful for commands like `drush @mysite.master uli`.

Unfortunately the Platform CLI doesn't distinguish between Drupal and non-Drupal applications - so it creates Drush aliases for any project - which is a problem for another day.