* Change order of linkify and preprocess; first preprocess and then linkify.

* Change order of linkify and preprocess; first preprocess and then linkify.
  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.