resolve #111: allow passing initial state to curried producer function

[pkerschbaum]

Should implement issue #111.

Notes:

• If a function is given both for fn and initialState, produce(fn, initialState) will throw an error, see https://github.com/pkerschbaum/immer/blob/202d54cb96949b45cdfdfb08f180b93723faef29/src/immer.js#L24.
  Because if produce would allow this, each call of the curried function with undefined would create a curried function again.
• The typings should be improved because currently, S is typed with any, and if two arguments are passed to produce, TypeScript (and I think also Flow) cannot exactly determine which overloaded version of the function gets called.
  If we add typings for the state/currentState/initialState S, the typing systems should be able to distinguish the produce calls.

[coveralls]

Coverage increased (+0.08%) to 96.296% when pulling 202d54c on pkerschbaum:feature/initial-state-for-curry-fn into 77c17f7 on mweststrate:master.

[Anmo]

Hi, I'm starting to test Immer (nice work @mweststrate 👍).

First, thanks @pkerschbaum for this PR.

But, regarding this issue, could I just give my feedback?
I would prefer to have a different method, instead of changing produce (even worse, when the original one already receive 2 parameters).

I would do something like (not necessary this method name 😛): produceWithInitialState(initialState, producer).

What you guys think about?

[mweststrate]

Great! Thanks a lot