Better handling for <a> tags for HTML output

[sandersk]

If possible, add param to support an HTML-processing mode (as opposed to XHTML-processing mode) for HTML outputs. In this mode, make sure that all <a> tags are marked up as <a></a> rather than <a/>, as the latter causes issues when browsers are parsing in HTML mode (not an issue for EPUB, which always parses as XHTML).