XML / RSS Output as an option for the request API

[bagobones]

A number of client tools can consume XML/RSS feeds for lists. As an option to make generic integrations directly to some tools it would be nice if the request API could output XML/RSS in addition to the default JSON.

For example Flexget can consume both rss feeds and search based rss feeds. Writing a full plugin for flexget would be best but there are other tools that would likely benefit from another generic output of the request lists.