Naming: what's a consumer?

[jonathonherbert]

A comment from @SHession in #166 raised the question – what's a consumer?

I suspect it's no longer a very good name. The best answer I can give is that a consumer is the thing that consumes state updates from the PME library. I suspect it's sort of vestigial after the work in #29, in that this interface is the responsibility of the ElementSpec to manage, and no longer has anything to do with the plugin interface.

Let's find a better name.