Add new instance variable, space_to_underscore (wikitext, adfcb69)

This instance variable will permit the creation of MediaWiki-style links, where [[foo bar]] is a like to "/wiki/foo_bar" rather than "/wiki/foo%20bar".

The benefit is prettier URLs; the downside is that [[foo bar]] and [[foo_bar]] now point to the same article. As this is a trade off it will default to off by default.

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