Add libhtml

libhtml provides escaping/unescaping of HTML entities. It matches the
HTML5 parsing rules as closely as possible. It provides convenience
functions to escape/unescape, helpers to perform the escaping/unescaping
during Show, and Writers that can escape/unescape in a streaming manner.

References:
http://www.w3.org/html/wg/drafts/html/CR/syntax.html