initial attempt at event handling.

// @sindresorhus @CWSpear 
Ok, two things I would like to mention. First of all a breaking change like this would require a **major** version bump if we are really following [semver](http://semver.org/).

I would also like to give my two cents on the events syntax problem. Why not consider using [js-signals](http://millermedeiros.github.io/js-signals/)? It is battle-tested library and has many advantages over the standard event emitter. It would allow us to have an elegant syntax with no breaking changes, something like:

```js
var config = {
  onFileInjected: new Signal()
};

config.onFileInjected.add(function (file) {
});

wiredep(config);
```