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?
Can add `use format_util::get_short_hash;` and `use git::get_last_commit;` in the imports to get this line shorter:

`let version = args.flag_setversion.or_else(|| get_short_hash(get_last_commit()[]).to_string();`