Add support for version compare and for checking for version part argument

[arcanefoam]

Version compare is important for other scripts that use bumpversion as a library, e.g. in my big osgi project I want to make sure all projects are at the same version, find which ones are ahead/behind.

I also took another approach to checking the version part argument. My implementation checks against the current VersionConfiguration (not hardcoded to specific names).

Both changes have proper tests (I think) in place.

[arcanefoam]

Sorry for the delay on this. Have pushed the requested changes, except for the equals (see my comments).

[c4urself]

@arcanefoam any movement here?

[arcanefoam]

@c4urself sorry for the radio silence, had been working on unrelated projects. I agree with most of the comments from @ekohl, I will reply to them and commit some changes during this week

[florisla]

This branch needs a rebase.