Allow start/end dates in a range to be null so there can be an 'All Time' range option

As originally described in #284,

Use case: I'd like to have "All Time" be a range, but want to avoid hardcoding some date like 1/1/1900. 

```
ranges: {
    'Today': [moment(), moment()],
    'This Week': [moment().startOf('week'), moment()],
    'This Month': [moment().startOf('month'), moment().endOf('month')],
    'All Time': []
}
```

The callback just returns `null` for start/end dates if this is chosen.