Refactor fdoc_to_html

Most modern Ruby CLI apps use Thor to build out their frontend.  Thor provides a lot of nice messaging and convenience for building a generator.  Given that fdoc has a CLI and it's a generator, this seems like a great fit!

The important bits:
* Renamed the executable from `fdoc_to_html` to `fdoc`, with a single action called `convert`
* Split the url base path parameter into an optional flag called `-u/--url-base-path`
* Split the output path parameter into an optional flag called `-o/--output`
* Default the output base path to `../html` based on normal usage, so for `docs/fdoc` it generates `docs/html`