Add plugin clean up hook and dispatch handler

[dudk]

Currently plugin doesn't implement any clean-up. What needs to be freed:

• Plugin double and float buffers (allocated by library);
• Plugin structure placed into map (done by library);
• Plugin internal resources (allocated by plugin code) - important note is that plugin developed in Go and go has GC, so most users won't have any recourses to free;

Because resources allocated by both a user and a library must be freed, it's not enough to just provide a hook for the user.