Support Immutable Store

[sosuren]

Field's value are not changed when using Immutable Store.
Probably allow using Immutable store: redux-form-compat and redux-form-compat/immutable ?

[aij]

Hmm, we don't use the immutable version of redux-form anywhere, so I'm not sure exactly how it's supposed to work. Nor do I have a bunch of code to test it with... PRs are welcome though. 😃

I'm guessing it would make sense to mirror what redux-form is doing, though I haven't looked too closely.