Fixed #44

This does involve altering the interface for plugins, each plugin has been updated accordingly.

@tokejepsen, if you've already started working on #49 (which will probably also change their interface) don't sweat it, I'll add my changes to your commit when you're ready.

I also had to disable `validate_mesh_history.py` due to issue with acting on an object set as opposed to meshes. I didn't look too closely, but I would assume a pre-flight check is in order. This also raises an interesting question, how do we deal with temporarily enabling/disabling plugins? At the moment, I've appended an underscore to it.