Polish feeds

[eevee]

trafficstars

• [ ] Still need to figure out how to make a feed "per comic". Or per chapter? Or whatever?
• [ ] I don't really like that I have to hardcode the whole feed <link> tag; seems like that should be injectable, somehow. (But then some pages might want multiple feeds, too.)
• [ ] Really want some cool resource_url defaulting in here.
• [ ] Flesh out more of the optional useful stuff. (How do I make optional stuff with z.i? Just let it return None, or inherit from the interface...?)
• [ ] See if any of these ideas can be pulled into shared code or patterns or something.
• [ ] Document the crap out of it all.