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

* `[[!preprocessor directive]]`
* `{{preprocessor directive}}`

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:

* The inability to use spaces in wikilinks or link text
* The requirement to use a trailing space on a preprocessor directive
  with no arguments, such as `\[[!toc ]]`

--[[JoshTriplett]]

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