2.0 Rewrite: COULD HAVE features

[JKrag]

These are feature that are deemed as could have for now. Consider this a backlog of ideas.

Some of this actually existed in the 1.x series, but is not deemed super important for now.

Usage:
  #{__FILE__} --latest=<tag-glob> [<BASE_VERSION>] [options]
  #{__FILE__} <ref>               [<BASE_VERSION>] [options]

Arguments: 
  BASE_VERSION A version string in M.m.p form that will be used as base for calculated increment. 

Options:

  --postfix-separator=<postfix-separator> 
      We assume that postfix is separated with '-' as per semver.org, 
      but you can specify a different separator char with this option.

  [--default-major | --default-minor | --default-patch ]