GitLab

Add `brew cleanup` command to `bubu` alias

sorin-ionescu had some good points.

Merge completion with official brew completion. Add a helper to link official
completion into oh-my-zsh plugin (without overwriting). Add an alias to list
installed brews. Add brews to the path (in a somewhat strange way).

Reorganizing plugins so that each plugin has it's own directory now so that any plugin-specific functions can be bundled within there.

- update plugin architecture
- completion function in $ZSH/functions/brew
- plugins/brew.plugin.zsh only activates