Is your feature request related to a problem? Please describe.
We have an issues-only repository on github which we use for tracking bugs relating to our game, The Last Starship

https://github.com/Introversion-Software/LastStarshipIssues/issues

There is no source code for the game itself here.

Every 6 weeks we work towards a new update of the game, and we create a Milestone in the issues to identify the bugs that we are working on and will hopefully fix for the next release.

Describe the solution you'd like
What I would like is github-changelog-generator to be able to list just the issues closed for a specific milestone. For example, a new command line argument --issues-for-milestone="Alpha 4"

Describe alternatives you've considered
I'm happy to do the coding for this, but I'm not quite sure about where to add it in in the code. Any pointers would be much appreciated.