Adding new plugins to WP Super Cache to extend it is complicated but
where they should go should be easy. This patch makes the plugin check
in a particular directory if it exists and load the PHP files there.
The files there won't be deleted when WP Super Cache is updated.

Add helper functions to add and delete plugin files from the list to be
loaded by WP Super Cache:
wpsc_add_plugin( $file );
wpsc_delete_plugin( $file );
And this function to return the list of plugins:
wpsc_get_plugins();