Github: allow filtering incoming builds (so you can ignore some branches)

[danielbeardsley]

We allow filtering branches on the outgoing side (git-build can be configured to only pull builds from the queue it's interested in). If you try to ignore a subset of branches, they will just hang around in the queue forever, never getting "built".

If you want to ignore some subset of branches / repos, that's not really possible right now.

Do this: Allow providing a filter function in the github config section and provide the repo + branch as arguments.