propose using cc for audience targeting. That's what it's there for

I found this post from @jasnell, the spec editor, from October 2013 discussing the subtleties of audience targeting when the set of users you are targeting is large, and maybe a subset of a larger community. Very relevant to our needs.

https://groups.google.com/d/msg/activity-streams/Tg9EJUVa3-o/LvETeZ3Q_BQJ

I think we are doing essentially the same thing here, but instead of the 'cc' audience including `urn:social:interested`, its the followers of a given topic in Livefyre.

This pull would switch to cc from our contrived 'context' key, which is a little too close to '@context' in [JSON-LD](http://www.w3.org/TR/json-ld/#the-context) (IMHO)

@jayv, @jasnell, do you agree?

Quick chat with @ninowalker leads to the supportive (for him) conclusion that this approach is "not one I can find immediate fault with" haha
`urn:livefyre:livefyre.com:topic=57572:subscribers` -> `urn:livefyre:livefyre.com:topic=57572:subscribers`?