* include/*,src/*: New plugins cache feature for faster load times.

* include/*,src/*: New plugins cache feature for faster load times.
   This is still experimental so isn't enabled by default (use --plugins-cache).
   There are also a few issues that aren't dealt with yet:
     - translation of module strings.
     - config options callbacks and actions.
     - a few memory leaks.
     - and likely other things ;)
   You can easily reset the plugins cache with --reset-plugins-cache.