Make plugin addressable from by others

In [my comment on another thread](https://github.com/avryl/wp-front-end-editor/issues/74#issuecomment-67035300), I ask:

> I'd like to ask that the plugin loads into a global variable, so that it's hooks can be addressed - it's really complex to address anonymous objects

This PR implements that. As a result, other plugins can control if WordPress Front-end Editor loads without resorting to miscellaneous hacks.