Fixes #390 - Merging of :fullscreen rules

Had to change the standard regexp for special selectors, hopefully this is ok :)
No typo here, from MDN:

> The W3C proposal uses the single word :fullscreen, that is without a dash, but both the Webkit and Gecko experimental implementations use a prefixed variant with two words linked by a dash: :-webkit-full-screen and :-moz-full-screen, respectively.