p5-HTML-Gumbo: add version 0.18

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.

(markd)