Add/array support

adds support for arrays as `options.evens`, for example:

```js
events = [
  { key: 'event', value: 1 },
  { key: 'event', value: 2 }
];
``` 

in this case doing `.track('event')` will track twice, once for `1` and then for `2`.

cc @ianstormtaylor @lancejpollard