Added support for RTF, HTML, and custom data formats

Added support for RTF, HTML, and custom data formats. :boom::clipboard::boom:

Fixes #143.
Fixes #152.

@jonrohan: Unsurprisingly, the SWF size went up again: 3.304 KB &rarr; 4.267 KB. (And 48 hours ago, it was 2.496 KB.)  We'll definitely have to try our best on #395. :-1: