Refactor fdoc_to_html to be a Thor CLI

* Use Thor to build command line interface
* Rename command from `fdoc_to_html` to `fdoc`
* Invoked with `fdoc convert (path) [--output=x] [--url-base-path=y]`
* Required output parameter is optional, defaults to "(path)/../html"
* Optional output location and url base parameters are now flags