Add support for <wbr> tag.

The current implementation does not appear to be aware of the \<wbr> tag.  This is the "word break opportunity" tag, which is processed as a void element.  Without proper recognition of the \<wbr> tag, htmlmin unfortunately adds illegal closing tags when processing HTML files that use it.

See: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/wbr

This pull request proposes a possible enhancement to support \<wbr>.