Add track helper

proof of concept:

```js
import Ember from 'ember';

const {
assign,
inject: { service }
} = Ember;

export default Ember.Helper.extend({
metrics: service(),

compute([ event ], params = {}) {
return () => this.get('metrics').trackEvent(assign({ event }, params));
}
});
```

Usage:

```hbs

Stuff

```

cc @offirgolan