www/p5-HTML-Gumbo: create port

Gumbo is an implementation of the HTML5 parsing algorithm implemented as a pure
C99 library with no outside dependencies.

Goals and features of the C library:

• Fully conformant with the HTML5 spec.
• Robust and resilient to bad input.
• Simple API that can be easily wrapped by other languages. (This is one of such wrappers.)
• Support for source locations and pointers back to the original text. (Not exposed by this implementation at the moment.)
• Relatively lightweight, with no outside dependencies.
• Passes all html5lib-0.95 tests.
• Tested on over 2.5 billion pages from Google's index.