add support for unhighlighting highlights with custom classes and/or tagNames.  this resolves https://github.com/jbr/jQuery.highlightRegex/issues/7

$highlight('#content').highlightRegex({ tagType: 'span', className: 'customhighlight' }) removes a highlight that was added by highlight('#content').highlightRegex(re, { tagType: 'span', className: 'customhighlight' })