Add createReducer

[AmaranthineCodices]

This fixes #7.

It adds a createReducer function that works effectively identically to the redux-create-reducer library. Example usage (taken from the tests):

local reducer = createReducer({
    a = 0,
    b = 0,
}, {
    a = function(state, action)
        return {
            a = state.a + 1,
            b = state.b
        }
    end,
    b = function(state, action)
        return {
            a = state.a,
            b = state.b + 2,
        }
    end,
})

The first argument is initialState, which will be returned if and only if the state provided to the reducer is nil. The second argument is a map of action types to reducer functions; the reducer corresponding to the action type will be used. If the action type is not in the map, the composite reducer will return the same value of state that it received.

[coveralls]

Coverage increased (+0.08%) to 99.029% when pulling 3b4c525 on AmaranthineCodices:createReducer into 9efe558 on Roblox:master.

[LPGhatguy]

An interesting side-effect of createReducer is that it assumes the shape of your actions (that they have a type field) -- is that something we should be comfortable nailing down?

[AmaranthineCodices]

Redux has an API requirement that actions have a type field. I think this is a reasonable API constraint to impose - without a field to denote the type of action, there isn't much of a point to actions. Naming the field type seems logical enough.

It might be helpful to throw an error in dispatch if the action does not have a type field.

[LPGhatguy]

Oh wow, I didn't realize that or notice it the last time I skimmed the documentation. Cool!

That check can come in a later PR if you're up for it, that would be a nice lint to have especially with middleware.