Make ImmutableJS optional

[devvmh]

This is not high priority.

It would be nice to either

1. Create the option of ImmutableJS, seamless-immutable, or no immutability.
2. Migrate from ImmutableJS to seamless-immutable.

[devvmh]

After trying this out on a branch, I've come to the conclusion that this isn't worth it.

If anyone posts this as a request in the future (most likely to save on the library size), perhaps I can switch to seamless-immutable or react-addons-update.

As it is, because immutable is wrapped by selectors, it won't "infect" user components.