Add tokenization of PRE_START and PRE_END (wikitext, 940a83b)

As a convenience, it would be nice to offer the ability to use literal <pre></pre> tags to mark up PRE spans rather than just using the existing mechanism of prefixing each line with a space. This will be particularly useful when pasting in long code samples and the like.

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