Add branch filtering to r10k

r10k issue # 574 is a request for branch exclusion for r10k. This means that on a "deploy environment" call, any branches matching some filter would not be deployed.

Open question: Would that mean that ```r10k deploy environment specificenvironment``` would deploy it, even if it matched? Hrm.

(EDIT: The original implementation was a regex setting. This was deemed too difficult to troubleshoot in practice, so we moved to a list of branch prefixes.)