This allows passing a wikilink inside a parameter to a preprocessor
directive without it being expanded to html, and leaking out of the
parameter, which had required some non-obvious use of triple-quoting
to avoid. Note that any preprocessor plugins that output something
that looks like a wikilink will now have it treated as such; AFAIK
this doesn't change any behavior though except for the template plugin.
* Enable preprocessor directives when previewing an edit.