Oembed proxy

This pull-request introduces `oembedProxy` option for embed add-on. 

This option, when configured, will receive URLs and parse it through the given server, then return [oEmbed](http://oembed.com) object with `html` as a field, if any. 

This approach to embeds is user-friendly as it 
a) allows for easy extending of the embeds coverage; 
b) doesn't lock user to specific implementation, but rather relies on any proxy that supports oEmbed standard;
c) isn't intrusive as when `oembedProxy` isn't configured, the embed add-on still functions as it used to do. 

See demo with `oembedProxy` configured here:
http://itteco.github.io/medium-editor-insert-plugin/

[Iframely](https://iframely.com) provides a courtesy oEmbed endpoint for this project at http://medium.iframe.ly. This one is meant only for dev and testing purposes and is expected to be replaced with the proper one (with `api_key`) before people go into production.

If you'd like to activate`oembedProxy` as the default option , please, uncomment line 11 in `medium-editor-insert-embeds.js` file. It will point default proxy to Iframely's courtesy endpoint. In this case, please, update readme file too to warn people to change proxy for prod usage. 

P.S. Iframely provides responsive embed codes for over 1600 domains...