`brew fetch` is one of the more often used commands but
for some reason is one of the few commands that doesn't autocomplete.
This just fixes that up so `brew fetch` gains magical autocomplete
powers along with the other commands. Nothing fancy.
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).