ikiwiki/ todo/ firm up plugin interface

I don't want this interface to be too firm; it's ok for a plugin like ddate to redefine an internal function like IkiWiki::displaytime if it wants to.. But plugins that still access stuff through IkiWiki:: should be aware that that stuff can change at any time and break them. Possibly without perl's type checking catching the breakage, in some cases. Plugins that only use exported symbols should not be broken by future ikiwiki changes.

Functions used by only some plugins, undecided about exporting:

Variables used by plugins but not exported yet:

done