Add a prefix_directives option to the setup file to turn this syntax
on; currently defaults to false, for backward compatibility. Support
optional '!' prefix even with prefix_directives off, and use that in
the underlay to support either setting of prefix_directives. Add NEWS
entry with migration information.
* Plugins can add new directories to the search path with the add_underlay
function.
* Split out smiley underlay files into a separate underlay, so if the plugin
isn't used, the wiki isn't bloated with all those files.