option to add link to video on youtube, useful for RSS feed visability.

Jonnas, I was using your plugin and ran across the issue of the embed not showing up in rss feeds and other generated xml files. I have added the option to the plugin for the user to add a text link to the video on youtube in the event they have regular use for xml files.

![screen shot 2014-04-21 at 9 01 29 am](https://cloud.githubusercontent.com/assets/4931974/2754454/191162ba-c955-11e3-9c93-74ea99dafb48.png)