Document Atom feed infrastructure

[jaredkrinke]

I would like to incorporate an RSS feed eventually, so it may be worth considering then.

Originally posted by @ghbug in #27

[jaredkrinke]

Used in both blog and md2blog themes, but not documented anywhere.

What's the right way to let other people build on this? It's a little tricky because it requires creating an item, adding metadata, and then using a special template. Maybe wrap it in a plugin? Or would that be too opinionated?