Implement better RSS Feeds

[Bios-Marcel]

Currently the feeds aren't iTunes compliant. It'd be nice to have that, as iTunes is one of the biggest podcast archives.
For this, we could probably use one of the following:

• https://github.com/mmcdole/gofeed
• https://github.com/eduncan911/podcast

However, I gotta see whether that lib also works for non podcast rss feeds. If it doesn't we'll have to use both libs and add a
switch to the config:

"RSSMode": "none|podcast|blog"

[Bios-Marcel]

Sadly github.com/mmcdole/gofeed is parsing only as it seems and github.com/eduncan911/podcast doesn't support chapters, which at least for me, is a requirement.