Add option --milestone to open_issue command

Closes #86, wanted to get an opinion on this before I finished up the feature with tests.
I was [trying to move away from prompts](https://github.com/education/teachers_pet/issues/8), but one solution would be that if they include `--milestone` without a value, it could list the milestones for the repository and ask them to choose which to use.

This leads to another question, which is how would the milestones get created across the repositories in the first place. Do we need an `create_milestone` command? This feels like :turtle:s all the way down, and while I appreciate all the work you've put into this, I'm questioning the value relative to this extra required effort.