improvement: hook system

[aam229]

I think it would be great to have a hook system in order to customize parts of the universal-redux module. This issue is not about which hooks we should provide but rather the architecture for the hook system. I suggest the following:

• Define an optional hook property in the config file. It should be a path to a file that registers the hooks.
• The hook.js file would look something like:

import { hooks } from "universal-redux"

export default function registerHooks(register) {
    register(hooks.server.CREATE_ROOT_COMPONENT, myfunction);
    register(hooks.client.CREATE_ROOT_COMPONENT, myotherfunction);
    ...
}

• Use the hooks in the universal-redux module:

import { CREATE_ROOT_COMPONENT } from "./hooks/server.constants";
import { hook } from "./hooks/hook";

const result = hook(CREATE_ROOT_COMPONENT, (arg1, arg2) => {
  //Default implementation
})(arg1, arg2);

I think it is important to have the default implementation defined in the file where the hook is called for better readability. However, I don't really like my syntax for calling it.

Please let me know what you think. I'm down to create a PR if we can agree on something.

[aam229]

This is part of #59