Introduces 'form.fetch' callback event.

This allows overriding the method used for reading form
templates. For instance, makes it possible to direct form
reads to flat files.

Closes issue #362

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5226 2fea6a4d-a838-0410-917b-93a53c48e9d2