Add ability to export Atom

[tommorris]

trafficstars

One of the benefits of the old microformats parsers was the ability to export the data out in a variety of useful formats beyond JSON: iCalendar, vCard, and Atom.

It'd be useful to have Atom output.

[snarfed]

fwiw https://granary.io/ (library and REST API) does this.

[capjamesg]

Closing this in favor of the precedent set in #71 that we should leave data munging to external libraries, rather than implementing that logic ourselves in mf2py.