Add option --milestone to open_issue command

Closes #86, wanted to get an opinion on this before I finished up the feature with tests.
This seems a lot better to me than trying to type out the raw string yourself.

This convention is also being used in [create_student_teams_spec.rb](https://github.com/education/teachers_pet/blob/a1cea0525e664d565bcff98c1256aa8713611fe7/spec/commands/create_student_teams_spec.rb#L22-25)