Add RSS and Atom serialization

[matthew-carroll]

This is a meta ticket for the goal of adding serialization capabilities to all supported feeds.

This package currently deserializes RSS and Atom feeds into type-safe data structures.

We'd like to add the ability to assemble those same data structures through code, instead of parsing XML, and we'd like the ability to take those data structures and serialize them to XML. This completes the RSS and Atom stories.

Here's a PR from @csells where he proposed such an API for webfeed: witochandra/webfeed#25

We should consider that API, as well as any relevant adjustments, and then implement serialization in this package.

• Choose an API approach
• Serialize RSS 1
• Serialize RSS 2
• Serialize Atom
• Update README
• Update Pubspec description

[matthew-carroll]

The following ticket specifically addresses adding a non-breaking serialization feature to the existing implementation: #46