ikiwiki/ todo/ New preprocessor directive syntax

As discussed on IRC, preprocessor directives should transition to a new syntax distinct from wikilinks. Possible syntaxes:

The transition would involve adding the new syntax, adding an option to turn off the old syntax with the default allowing it, giving people time to convert their wikis and turn the option on, and releasing a new ikiwiki (version 3 for instance) that turns off the old syntax by default.

Making this transition would fix two major warts:

--JoshTriplett

done in version 2.21, using the '!'-prefixed syntax. --JoshTriplett