Allow configuration of name of Mixpanel page viewed event

Instead of [hardcoding the event name `page viewed`](https://github.com/poteto/ember-metrics/blob/8e5ac364e23129ac69e4dfa6c05d3167ff4dabf7/addon/metrics-adapters/mixpanel.js#L65), allow this to be configurable via the ember-metrics configuration object.

example implementation:

```javascript
trackPage(options = {}) {
const config = get(this, 'config');
let { trackPageEventName } = config;
trackPageEventName = trackPageEventName || 'page viewed';
const event = { event: trackPageEventName };
const mergedOptions = assign(event, options);

this.trackEvent(mergedOptions);
},
```