Add optional "license" and "license_url" parameter

Provided information about media might require that some kind of license for that content is shown on the embedding page. Such license information is currently missing.

Example:
```
{
   "version": "1.0",
   "type": "video",
   "license": "CC BY 4.0",
   "license_url": "http://creativecommons.org/licenses/by/4.0/",
   ...
}