Allow --submit to login and submit to Gradescope

[cycomachead]

It would be nice if we could find a way to get this to work.

I propose than a assignment.ok file contain a JSON object:

"gradescope": {
  "course_id": 0,
  "assignment_id": 0
}

If this is present, then we ask the student to login a make a submission to the assignment.
If the endpoint is also specified, then it seems like we would still submit to okpy as long as it's running.