Add XKCDPlugin class with basic xkcd command.

Well, it's seems an XKCD API wrapper would be pretty low hanging fruit. There supposedly is an xkcd module on PyPI (https://pypi.python.org/pypi/xkcd/2.3.1) but it is so much un-pythonic (getRandomComic? really?) that the author did the right thing and removed the repository on GitHub altogether.