Generate releases between date range

It would be nice to generate a changelog within certain date ranges. It would be highly useful when using GitHub Release Notes as well. I was thinking of implementing it like so:

```php
// generate changelog with all releases (current functionality)
$changelog->generate();

// generate changelog with all releases after 2 September 2017
$changelog->generate(new DateTime('2017-09-02'));

// generate changelog with all releases up to 5 November 2017
$changelog->generate(null, new DateTime('2017-11-05'));

// generate changelog with all releases between 2 September 2017 and 4 May 2018
$changelog->generate(new DateTime('2017-09-02'), new DateTime('2018-05-04'));
```