Move to oEmbed Only

Because of the addition of customize, and the future move to async embeds, the Wordpress plugin can be drastically simplified to use the oEmbed endpoint only.

This removes the need for the AntiMangler library, which tries so hard to be smart and helpful but also causes lots of unintended problems.

This update is awaiting @MaxPower15 work on async embeds.

Still to be done:

* testing
* remove travis yml (unless new tests can be added?)
* update installation guide and bump version (**or** create new plugin altogether)