Change output format to xhtml, and enable --output-filename option.

XHTML format is better for post-processing than HTML, because many platforms/languages are lacking a decent HTML5 parser, but most of them have good XML parsers. XHTML is also required by Epub.

XHTML files have extension ".xhtml" or ".xht" but the default is ".html", so It is necessary to enable the `--output-filename` option.