JENKINS-17658. Add BFA_CAUSE token macro.

So this is still very rudimentary and almost certainly should be rewritten...and I couldn't figure out anything like an elegant way to write a unit test for it, so, well, I skipped that. =) But it's the guts of addressing https://issues.jenkins-ci.org/browse/JENKINS-17658 - obviously, I'd very, very much like to get feedback or see it all rewritten in a more sane way. It's just the output of about 30 minutes of work, after all. =)