Allow repos to be searched via Regex

[salemhilal]

As per #399, Hound instances with a lot of repositories can cause requests to become too large. While changing the verb from GET to POST might be a potential solution, it also means losing the implications that come with a GET request (i.e. that the request can be safely replayed).

A potential fix would be to allow repos to be selected via a regex instead of a list. I think this would need to be a separate request parameter entirely.