Add flag to manage plugin status

The flag whether this plugin is active or not.
Unfortunately, classic plugin stratedy does not have such feature.
Add API for external use

Extension provides a chance to extend a plugin but it is called by
plugin which provides it. It is not suitable to call plugin functions
at any time from external.

This patch provides API to call functions in this plugin from external.
Merge pull request #127 from rinrinne/plugin-api

Add API for external use