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?
Nope, I mean the official guide.

>However, it's considered better practice to not import individual functions, but to import the module, and only use one level of qualification:

https://github.com/rust-lang/rust/blame/9ca399f14e4a29d744ea3e8f04377a5569d338b3/src/doc/guide.md#L1597-L1607