Provide callback before setting state of the editor media manager

[griffinjt]

In the current setup, there is no way to modify the current state or views when the media manager is opened from the "Add Media" button. By the time you can modify anything, all the default views have been rendered and all of the events have been fired.

It would be helpful to check for the existence of a callback, and if it exists, run that before returning the workflow.

My patch does this, which you could invoke in this way:

`wp.media.editor.extend = function(workflow){

Manipulate the workflow here

}`

Thoughts? This is only for the default media manager that is invoked by clicking the Add Media button.