Add support for forcing a major/minor/patch bump

[gstelmack]

gotagger requires commits since the last tag to determine the new version. However, there are times when a new version is required without an actual commit, for example when the repo needs a rebuild to pick up fixes in dependencies but without any changes to the repo itself. Right now this is handled with dummy commits that update a README or some other trivial change, but we'd like to be able to get the rebuild done without an actual change. This can be worked around with a manual tag, but it is cumbersome.

This would be easier if there was a way to force gotagger to do a version bump regardless of the commit history. The immediate requirement is for a patch bump, typically minor or major only change when there is a real change to the repo, but it might be worth having those capabilities as well "just in case".