Support build metadata more explicitly

[ajoberstar]

Need to decide if there are some general things people would use for metadata that go across different Vcs impls. If so, should there be more methods on the Vcs interface? If not, should we just expose an interface to get an internal interface for a specific Vcs.

At the most general, there should just be a better spot to plug this in.

[ajoberstar]

Common things used for metadata:

• revision count (number of commits)
• revision id (commit hash)
• branch name
• timestamp

[ajoberstar]

Making it responsibility of the prereleasestrategy.