Add JSON version of RSS

{
          "title":"WebMention",
          "description":"A modern alternative to Pingback.",
          "link":"http:\/\/webmention.org\/",
          "source":{
            "url":"http:\/\/glea.ms\/u\/sull\/DevLog",
            "value":"@sull\/DevLog"
          },
          "category":[
            "indieweb",
            "pingback",
            "inReplyTo"
          ],
          "pubDate":"Tue, 23 Apr 2013 23:21:14 -0400",
          "guid":{
            "isPermaLink":"false",
            "value":"1366773674833"
          }
        },