Bugfix issue 22 new version

[jeremycarroll]

This addresses part should not be required if supplying --new-version #22.

Notice:

• it is backwardly incompatible, since the bug is with the command line API not the implementation of the API: this might merit a 0.6.0 version number.
• the tests change to reflect the change in API
• the implementation is short and direct but somewhat dirty, inspecting the argument list for --new-version directly. I find the cascade of ArgumentParsers unattractive, and this fix makes it marginally worse in my view.

Fixes #22

[ekohl]

You can ignore appveyor for now. #34 should fix that.

[jeremycarroll]

My plan is to wait for #34 to be merged, and then merge it into this

[ekohl]

Now that it's merged, would you mind rebasing?

[c4urself]

Let's rebase and merge this in and I'll cut a new release.

[jeremycarroll]

I am not working on this, and do not envisage doing so in the next two to three years.