Add Drip integration

This adds [Drip](https://www.getdrip.com) as a provider. We'd love to have Drip included in segment.io -- [here's our vector logo](https://www.dropbox.com/s/pv9b1zzagowgw4q/drip-logo-2color-cmyk.ai).

Quick question regarding our implementation. We have a conversion tracking feature that accepts a goal ID. In raw form, the call looks like this:

```javascript
window._dcq.push(["trackConversion", { id: 1234, value: 2000 }]);
```

I'm currently pulling that ID out of the options object:

```javascript
analytics.track("Trial Signup", { revenue: 20.00 }, { Drip: { goal: 1234 }});
```

Is this the best way to handle it?