Experimental: `smartContent` for some feed items.

Ask the feed account for an enhanced version of the item's content.

For instance, in github I'd like to see the repository description for
some kind of activities without opening the item in the browser.
Github should include it on the feed but it doesn't.

The implementation is a proof-of-concept (aka quick hack). 
It makes synchronous requests to github api (they are fast, but I guess it can lock the app in old machines or on a poor network).