Port Redux's todos example

[nickserv]

Fixes #29.

I wanted to add an example both for documentation and dogfooding. I think todos is a great example because it's already used in many places including the basic Redux docs, this package's readme, reselect's readme, and Redux's automated tests.

I think we should do the async example next since it's also from the Redux docs and it leverages redux-thunk. The more similar we keep this to Redux's original examples, the easier it is for users to compare setup with this package and for us to potentially merge this back into Redux's examples.

• Copy example from Redux to make a potential upstream merge easier
• Use this package instead of using Redux directly
• Use mutating reducers with immer
• Use FSA
• Use selectors
• Keep usage consistent with readme (now it's based more on the original example)

[nickserv]

I think this is ready! Here's an easier to read diff that just includes my changes to Redux's original example and our readme: nickserv/redux-toolkit@86bf0b6...example

[spsaucier]

It's unfortunate that this doesn't include any examples of createSlice or createAction.

[markerikson]

Yeah, this PR is way stale. Thanks for starting it, @nickmccurdy , but we probably oughta just close it and restart from scratch.