Handle <base> and specifying selectors to knock out (redux)

This now properly handles including URLs based on a <base> tag, and there's a new knockouts parameter (or repeatable --knockout command line option) that allows specifying CSS selectors to be removed from the output HTML. This could be useful for removing popups or ads, for example.

(Forgot to bump the version number in pyproject.toml, so this is a re-do of 0.4 with that fixed.)