feat(main): implement fallback for --setversion

This commit changes the behaviour so that if
no `--setversion` is specified clog will just
use the short hash of the current revision
as the version number.

Fixes #10

@Ryman do you like to review this first?
In reference to aturon's style guide? I think the preference for qualified functions is still considered 'RFC' and definitely isn't widely followed in current rust libraries (that I've seen) / rustc.

It's a lot easier to change the import line when it gets shuffled around in an external library than updating every call, but that could be less of an issue if you've got stable dependencies (eventually)!