_s: Use semantic versioning.

Fairly minor and noncommittal change. It does encourage theme authors
to think about semantically versioning their themes but doesn’t require
`_s` to take a strong stand on it.

It also removes an obstacle from submitting the generated theme to the
WordPress.org Theme Directory, as it refuses themes with non-numeric
version numbers.

Fixes https://github.com/Automattic/underscores.me/issues/8
Closes https://github.com/Automattic/underscores.me/pull/20
Props @ScottSmith95.