git_tag: Allow glob matching, multiple branch tracking

Mitigates #22 (closed) to a degree.

• Adds match and exclude attributes to source config for git_tag, which are lists of glob patterns. The plugin then uses git describe's --match and --exclude arguments to restrict the tags which are tracked to.
• Adds a track-extra attribute, a list of branches/tags to be tracked in addition to the track parameter, and updates the ref to the latest valid commit on either. This is intended to be used mainly in conjunction with track-tags in the event that upstream uses two branches - one for a current release and master.