Support for nested categories

[Adirael]

Hello,

As per the iTunes RSS spec a Podcast can have multiple categories, which can be nested on each other. There are two level of categories that can be nested within each other which is not supported by the current version.

I added suppor for this and the itunes:explicit tag. I saw two different pull requests implementing the itunes:category tag, but none was correct (one of them removed escaping altogether and the other one only supported a single level).

Would you be interested in a pull request regarding this? My change breaks backwards compatibility since it uses 'categories' instead of 'category' on the config, but I could whip something out to support both if you prefer it.

Cheers