Additional features to RSS / Atom feed generation

[csarven]

Follow-up features to generating a web feed (RSS / Atom) after #373 (comment)

• Adding resources to the feed from different containers or storages.
• Using a list-builder or dual-list box was one idea. (Or perhaps another way that's cleaner / economic in space / simpler?)
• Selecting a container adds all of its contained resources to the feed.
• (un)Select all resources in a container.
• Sending a notification somewhere that the feed is available. (Maybe overkill? Generally only useful when not already subscribed.)
• For resources without a representation including a concrete RDF syntax, try to build the feed item data from DOM parsing of HTML, SVG documents. That's when those documents do not include any RDF statement encoded in RDFa or data islands in script. This would for instance be useful for typical plain ol' HTML documents (without any RDF).