Support for dynamic selectors #18

I've noticed that the Selectors are all executed immediately and the results of them are provided to the React components for rendering. This means that it's difficult to have selectors that take parameters.

What I've taken to doing, which works but feels a bit strange, is having Selectors that return functions. For example:

    selectUsers: (state) => () => selectUsers(state),
    selectUserById: (state) => (id) => selectUserById(state, id)

So the React component sees two functions, one called selectUsers with zero parameter, and one called selectUserById with a single parameter. These can then be called as needed and will return the correct values from the state, but the actual execution of them is only on-demand and they can be parameterised.

I'm not sure if such a feature would be beneficial being included as a core part of redux-box, or how, but it would be useful for me :)