Remove --epub-stylesheet; just use --css

[jgm]

One problem with --epub-stylesheet is that multiple stylesheets can't be specified (#847).
Another is that it works by setting the css variable of templates (in the EPUB writer), which will have bad results if the --css option was used too.
Better and less confusing if we just allow --css to be used with epub, and get rid of --epub-stylesheet.

That means we can get rid of the writerEpubStylesheet field of WriterOptions.
The EPUB writer will look at variables for css. If no css files are specified in this way, we'll insert the default stylesheet as we do now. If css files are specified, they'll be copied to entries in the epub container, with appropriate names, and the variables will be changed to match the new names. We'll also have to change the code that contructs the manifest, so entries for all CSS files used get in.

All in all, not a huge change.

This replaces #847.