olark: add groupId, resolves #293

/cc @DirtyAnalytics @calvinfo

https://www.olark.com/documentation/javascript/api.chat.setOperatorGroup

just playing around with it, there are cases where it doesn't work out the way it currently is, so we should figure out a different convention imo.

- https://github.com/segmentio/analytics.js-integrations/blob/master/lib/facebook-ads/test.js#L68-L81

take this as an example (which happens often):

```js
analytics.track('a');
analytics.called(window._fbq.push, [ 'track', 'a' ]);
facebook.options.events = { a: 1 };
analytics.called(window._fbq.push, [ 'track', 1 ]);
```

the first is for arbitrary events, so it gets the _unmapped_ value, while the second is the _mapped_ value. it doesn't test the actual functionality if we do this:

```js
facebook.options.events = { a: 'a' };
analytics.called(window._fbq.push, [ 'track', 'a' ]);
```

it's even more confusing when you see two tests like this:

```js
// test 1
analytics.track('event');
analytics.called(window.lib.push, 'event');

// test 2
int.options.events = { event: 'event' };
analytics.track('event');
analytics.called(window.lib.push, 'event');
```

there are a few more cases that happen often that are consistent with this ambiguity too, can't remember of the top of my head.

so it would be good to create clear distinction between key and value, or event name and event value. so it would be cool to do this:

```js
// test 1
analytics.track('event');
analytics.called(window.lib.push, 'event');

// test 2
int.options.events = { event: '-event-' };
analytics.track('event');
analytics.called(window.lib.push, '-event-');
```

The `-name-` is just arbitrary, but is distinct from all other casing, so we know that it's the actual value coming through and not some sort of object key.