Make +X::Y+ be treated as typewriter text, same as <tt>X::Y</tt> (Fixes #319)

We could use a more specific regexp that only matches :: and not
just : inside the string, but I didn't think it was worth the
additional complexity.
Merge pull request #322 from jeremyevans/tt_constants

Make +X::Y+ be treated as typewriter text, same as <tt>X::Y</tt> (Fixes #319)