onVideoInsert event method (and onAudioInsert method)

[trymeouteh]

To add a custom event method which will be called when a user clicks the insert video button in the editor and pastes a video URL and clicks Save.

This event method will allow the user to parse the URL, determine if it is a valid video site such as YouTube, Vimeo, Dailymotion, etc and if so, can convert the URL into a valid iframe URL and return an string which is an iframe tag or return undefined meaning no video iframe is inserted due to it being an invalid video website.

This will allow devs to simply add support to their own video sites.

And to add a onAudioInsert event method for when an audio URL is inserted.
#4582