Add option --milestone to open_issue command

Closes #86, wanted to get an opinion on this before I finished up the feature with tests.
It's not a problem, if nothing else I had good learning experience out of it, and I can extract out the json code I wrote and make that into a new PR :smiley:.

I don't know how often educators use milestones in students repos. My guess would be not very often. Just by talking to educators the last few days it seems like it's a very rare case.