* create a public `initialize` method on the `wp.mediaelement` namespace
* make it idempotent to prevent nasty side-effects caused by initializing media elements more than once
Props bradyvercher.
Fixes#32423.
git-svn-id: https://develop.svn.wordpress.org/trunk@34346 602fd350-edb4-49c9-b593-d223f7449a82