A few fixes to inline styles and code blocks

This contains various fixes, including:

* Inline styles shouldn't work across blank lines.
* Inline styles shouldn't highlight if the close token isn't present.
* `[link text][destination]` blocks should allow a newline where currently only a single space is allowed.
* Fenced code blocks can start after blockquotes, like other block-level elements.
* Blockquotes shouldn't require the space.

I think this at least partially obsoletes #16.