Figure out complicated item fields

[freiheit]

• [ ] Handle stuff like content:encoded that gets turned into a dictionary.
• [x] Support more complicated field names like <itunes:summary> (this might already work)
• [ ] Model these more complicated fields/attributes in the config
• [ ] Support nested stuff in the items (accessible via item['outerField']['innerField']).
• [ ] Figure out how to get at interesting data when it is in an attribute of a field instead of contained by it (<enclosure url="..." />).