Refinements to cover handling

Refinements to cover handling:

* Better default handling for the generate.cover.html param, such that it's turned on by default if there's a `<figure data-type="cover">` in the source, but otherwise turned off. 

* Ensure cover.html-generated cover content preserves markup in source `<figure data-type="cover">`.

* Remove effectively extraneous param $metadata.cover.filename