Added changelog command

Added the changelog command to show the changelog information:

Usage:

````php
./gh changelog
Changelog since initial commit:
* feature #1 Added sha2pr command to show merge commit information (peterrehm)
````

````php
./gh changelog --range=d014ad0..HEAD
Changelog for reference range d014ad0..HEAD:
* feature #1 Added sha2pr command to show merge commit information (peterrehm)
````

Tickets:
- none