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
On first blush it looks reasonable... although, that's based on not knowing your exact use case. This does, however, appear to fall in line with exactly what the audience targeting properties were intended for.