Add HTML parser

Right now we only have an XHTML parser but it would be better to have an HTML one too (it would call the HMTL cleaner to generate valud XHTML and then use the XHTML parser).

This will allow users to choose the HTML syntax to enter HTML. It will also allow the new WYSIWYG editor to use it directly without having to call the HTML cleaner first and then call the XHTML parser.