Add session$userData feature

[jcheng5]

Fixes #1512

This implements session$userData as an environment, not reactiveValues. @wch @bborgesr feel free to weigh in. I could go either way.

[bborgesr]

I vote for leaving it as an environment. Joe, in #1512, you said:

Only question I have is whether session$userData should be a regular environment, or a reactiveValues? (If the former, if you needed reactivity you could create a reactiveValues object and put it into a slot in the regular environment; but if the common case is needing reactivity it might be nicer to do the latter.)

Your parenthetical is good enough for me. I think if people need it, they should go through the trouble of doing what you suggest. The reason I don't think this should be the default is because I think that, in most cases, this will be used just for counters and regular objects, in which case, it would be a pain to have to do isolate(session$userData$counter) each time you want to access the value without taking a dependency on it (or even when you're outside of a reactive context -- if you're just accessing the value from a regular function, for example).

[wch]

I think using an environment is a good idea.