Update blocks to better match the spec

[GuiltyDolphin]

As mentioned in #16, QUOTE blocks currently don't allow markup within them, but according to the spec they should.

According to the spec, there are two "groups" of block: greater blocks and block elements.

These parts of the spec outline certain special types of block, and make it clear which blocks support extra keywords, which can contain markup, etc.

I'll have a look into this soon, but I don't think it'll be too difficult.