Create site config system

[ianb]

TowTruck should be somewhat configurable by sites. No configuration should be necessary, but it should be possible to override hubUrl, maybe indicate other hooks, etc.

I imagine something like require.js (after #10 is implemented), where you could either do:

TowTruck.config({
  hubUrl: "something else"
});
// or like require.js, if TowTruck hasn't been loaded:
TwoTruck = {
  hubUrl: "something else"
});

[ianb]

While hubUrl has to be configured early on, and maybe a few other things, I think for other hooks something more like an event system will make more sense.

[ianb]

Implemented