The `/feed` endpoint can now return the feed as JSON

As of server v0.5.32, /feed takes a format parameter: format=xml (the default) returns the RSS document as always, and format=json returns the same feed translated into JSON. The structure is rss.channel.item, the names are RSS 2.0's own names, every element -- source:markdown, source:inReplyTo, source:comments, all of it -- exactly where the XML puts it. An unsupported format name gets an error naming the two real ones.