Allow outputting RSS, Atom, and JSON feeds

[cdzombak]

This PR allows gofeed to convert the universal Feed representation back into RSS, Atom, or JSON feed structures (performing the inverse of translators) and render those structures out to RSS, Atom, or JSON feeds.

• closes Feed creator (XML writer) #33
• closes XML annotations on the structs #263

[infogulch]

This is pretty neat! How would you expect the server to handle pagination? Say Feed data is in a db, and the http request is for page 2. First query the data for that page, then populate the Feed struct, then pass that to the Renderer. Does that sound right?

[mmcdole]

Thank you for the contribution! I'll look through the changes this week hopefully.

One thing to consider is if we want to have these changes target the v2 branch or not.

[cdzombak]

One thing to consider is if we want to have these changes target the v2 branch or not.

🤦🏻 I was totally unaware of the v2 branch. Sorry about that.

[PhantomPhoton]

Thank you @cdzombak for the PR. I'm using it very successfully for a home project I have. My $0.02 is that it's worth landing into the v1 branch even if it would need work for v2, as it's useful as is and v2 isn't stable yet.

Really appreciate the project as a whole! Thank you all and hope you all have a happy new year!