initial attempt at event handling.

// @sindresorhus @CWSpear 
It would indeed be a breaking change. We can just bump the minor to not break anyone's setup. However, I tried typing out what it would look like, and it's not very appealing:

```js
var wiredepThisFile = $.wiredep({
  src: __filename
});

wiredepThisFile.on('file-injected', function (file) { /*...*/ });

wiredepThisFile();
```

I don't know which is worse... passing in your own event emitter, or this.