Support for module:content 

I added support for <content:encoded> that is pretty widely used to integrate HTML content in RSS feeds.

I'm currently using it in LearnByHacking. See for example https://www.learnbyhacking.org/posts
and the corresponding code: https://github.com/scslab/lbh

I can add support for ther other stuff mentioned in the module (e.g., <content:item>), but I don't think they're as widely used as content encoded.

[Sorry for bumping the version, I can drop it if you wish]