Make plugin addressable from by others

[christianwach]

In my comment on another thread, 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.

Replaces #205

[solhuebner]

:+1: to get it merged :)

[ellatrix]

Change "wordpress" to "wp" and okay.

[christianwach]

Thanks @iseulde - done.