external links should use underscores instead of spaces

Previously:

Input: `[[Foo Bar]]`
Output: `<a href="Foo Bar">Foo Bar</a>`

Now:

Input: `[[Foo Bar]]`
Output: `<a href="Foo_Bar">Foo Bar</a>`

Note that the relevant part of the spec:
http://wikicreole.org/wiki/Creole1.0#section-Creole1.0-LinksInternalExternalAndInterwiki
gives examples that show this behaviour.