Create a redux reducer from an object of action handling functions, keyed
by the actions they handle. Has createReducer.compose(fns) helper function
to combine multiple action handlers into a single action.
Big thanks to dlmanning, our conversations lead me here.
var createReducer = require('create-reducer')
// setup handlers, object keys for action types
var handlers = {
  add: add,
  multiply: multiply,
  // compose handler functions together for compound actions.
  addThenMultiply: createReducer.compose(add, multiply)
}
module.exports = createReducer(handlers, function () {
  return { num: 0 }
})
// actual action handling functions, down out of the way.
function add (state, action) {
  return {
    num: state.num + action.payload
  }
}
function multiply (state, action) {
  return {
    num: state.num * action.payload
  }
}