Add a bunch more PRE specs (wikitext, 48a3f27)

I'm checking as many weird corner cases and exceptions here as I can think of.

The basic behaviour is to allow a literal <pre> only at the very top level or inside a P block, as long as the latter is not inside a BLOCKQUOTE block (this is because I'll later be adding literal <blockquote></blockquote> support and I don't want to allow intermixing the two markup styles to avoid confusion). Any open span-level elements are automatically closed whenever such a <pre> block is entered, including rolling back any unfinished internal or external links.

If an unexpected <pre> or </pre> token appears anywhere else, echo it in escaped form to provide feedback to the user; I think this is better feedback than just autoclosing the previous spans/blocks and then starting a <pre> block anyway.

Signed-off-by: Greg Hurrell <greg@hurrell.net>