Ensure handling of missing opening/closing tags for all HTML5 elements
Right now the lexer contains a set of rules that can be used to close certain elements that are left open, based on a newly found opening tag. These rules are mostly based on those found at http://www.w3.org/TR/html5/syntax.html#optional-tags but this list doesn't contain the rules for all elements (how convenient). As such it's best to review every single HTML element and add any missing rules.