Feature: Add mediaelement.js and video embed documentation.

## Ready State: Yes

This PR adds mediaelement module and library to support html5 video and audio tag usage across browsers. This is important because the media browser preview and ckeditor use the video tag. It's also important because it makes possible theming of the video player by removing the need for the video to be embedded in an iframe. However, this must be configured to do so using the file type display view modes.

This PR also adds documentation to the README to illustrate post-install configurations that need to be made to support a seamless video embed setup. The choice was made to leave these as post-install steps because storing or building defaults to these configurations is not easy and in some cases undesirable. 

## Requirements & Dependencies

* None

## Deployment Instructions

* Merge code.
* Build and install `mediaelement` module and library
* Clear caches with `drush cache-clear all`

## Usage/Test Instructions

* Make sure to turn on mediaelement sitewide configuration if you're not installing or reinstalling imagex_media.
* Follow the configuration instructions for the video file type view modes.
* Add a video through the web tab in the media browser and make sure the video embeds.