support Facebook embedded posts as shortcode and embed URL

Support Facebook's new [Embedded Post social plugin](https://developers.facebook.com/docs/plugins/embedded-posts/) markup.

Introduces a new [shortcode](http://codex.wordpress.org/Shortcode), `facebook_embedded_post`, accepting a single required parameter: `href`.

The plugin will attempt to recognize possible Facebook post URLs capable of display inside the Embedded Post social plugin and register each URL pattern through [wp_embed_register_handler](https://codex.wordpress.org/Function_Reference/wp_embed_register_handler).

Embedded Post display is limited to [Facebook public content](https://www.facebook.com/help/203805466323736/) and may be initially limited to specific Facebook partners. The plugin acts as a helper for URLs that could be Facebook content URLs by passing the values to the Facebook SDK for JavaScript for interpretation and possible rendering of post content.