Add version argument

[sopel]

This closes #52: Aside from the obvious addition of a --version argument, it is implemented via python-versioneer, which is a tool for managing a recorded version number in distutils-based python projects:

The goal is to remove the tedious and error-prone "update the embedded version string" step from your release process. Making a new release should be as easy as recording a new tag in your version-control system, and maybe making new tarballs.

• Version String Management in Python: Introducing python-versioneer provides a more detailed overview of the problem and approach

This seems to work nicely, despite issues being on file for Windows (likely those depend on the specifics of the local Git installation, they do not seem to apply to me at least) and does indeed remove the tedious and error-prone "update the embedded version string" step from your release process, besides nicely surfacing commit based and dirty builds; it isn't out of the question that this solution needs to be reconsidered though.

[le-bott]

Merged build started.

[le-bott]

Merged build triggered.

[le-bott]

Merged build finished.

[sopel]

:information_source: I'm going to merge this despite the failing build, which is caused by #53 and triggered this iteration in the first place - it will be tackled next accordingly.