Move DistilledPost typo to Hakyll.Convert.Common.
Introduce notion of categories.
Move some blogger-specific code to Hakyll.Convert.Blogger.
Bit of commented code for manipulating content.

Turns out this manipulation wasn't necessary; I was mistakenly trying
to pass my HTML through Pandoc instead of leaving it intanct like I
originally wanted.  But the descendElem thing seems useful to keep
around.
Support Wordpress, tidy structure a bit.
Document the fact that we support Wordpress.