Pending Agreement: Add Support for Epoch Semver

[jtannas]

PR to add support for the feature proposed in #113

Putting it up early as a proof-of-concept and as a demonstration that I'm willing to do the work.

There's some comments in it that should be reviewed but otherwise I don't see much more work to be done.
$ bundle exec gem bump -v epoch --no-commit behaved as expected.
$ bundle exec gem bootstrap tmp behaved as expected.

Leaving it as draft until there's agreement to add the feature so that discussion stays in the linked issue.

P.S. You'll probably want to squash merge my commits since there's a lot of "guess and test" going on.