feat(Store): Allow parent modules to provide reducers with tokens

[MikeRyanDev]

Also allows feature modules to declare just one reducer function instead
of a complete action reducer map.

cc @vsavkin

Closes #34

[bbaia]

Also allows feature modules to declare just one reducer function instead
of a complete action reducer map.

Why only feature modules ? why not root module too ?
If not, you should throw an error or make TypeScript fails if you provide a reducer function to the forRoot method.