Added <<else if>>

* Updated <<if>> to support <<else if>>, which essentially allows
multiple <<if>> macros to be chained together without nesting. I'm sure
you understand.
* This also eliminates a few implicit globals that
macros['if'].handler() was creating when it ran.
* Added 'to' as a Twine macro parameter keyword. This is converted to '
= ' by Wikifier.parse(). I noticed a few people on my Twitter feed were
getting confused between 'eq' and '=', so hopefully this will serve to
disambiguate them further.