Tab completion for `brew tap`

Using an inline Ruby script we can hit the GitHub API and look for
repositories that match the tap naming scheme. The results are cached
for the duration of the current shell session, so going over the network
is a once-per-shell-session cost.

There are a few false positives, but not much we can do about that at
this point, as taps do not have to be in the fork network of any
specific repository.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>