Add a Youtube widget

There are still some commits to come to get it to work with our new development environment,
namely that the plugin is being loaded from outside of the ckeditor plugins folder, such as updating `test.html`.
Shouldn't be difficult. It would likely mean splitting the plugin into 2 separate files, something like:

- videoSearch.js
- videoSearchYT.js

Where the former assumes a generic framework/object for the result data for outputting the results, and the latter converts the json pulled from YouTube's API into that required structure. It's also be helpful for easily turning this into the Vimeo search tool, which I'm guessing is what you were getting at.