When running `brew livecheck --cask` or `brew livecheck --formula`,
livecheck wasn't properly respecting these flags. It should have
worked by only including the casks or formulae in the watchlist but
instead these flags were treating all the names in the watchlist as
formulae or casks, which doesn't work properly.

This addresses the issue by always using `#to_formulae_and_casks`
on the watchlist names and then using `#reject` to conditionally
exclude formulae or casks based on whether the related flags were
passed to `brew livecheck`.

- Skip blank lines in watchlist
- Initialize Livecheck#version as nil
- Simplify livecheck_version logic
- Make test a bit more understandable

Co-authored-by: Markus Reiter <me@reitermark.us>