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?
true, but then the official guide says it's discouraged to import individual functions and rather only import until the parent module so that it becomes `module::the_function()`.