Functional Reactive Feeds

I believe that this is how plugins on the web will work:

1. Say you want to spell check a feed of entries.
2. You add the spell checker service as a subscriber to your source feed.
3. The spell checker publishes a results feed with one result entry for each source entry, correlated via the source entries' IDs.
4. Your authoring tool subscribes to the results feed and displays the matching result entry directly below each source entry.

The display options are of course highly app-specific, and we'll want those feeds to be realtime enabled.