Customizable student ID in nbgrader submit

[nthiery]

This implements nbgrader submit ps1 --student a4.jane.doo

Some use cases:

• handling groups of students: helping the instructors access just the students of their group by adding a common prefix
• submissions directly from a jupyterhub, where all students share the same jovian login; in general infrastructures where all students share the same login
• a posteriori submissions by the instructor (e.g. because they are collected by some other mean)

[jhamrick]

Thanks!

As discussed, this will make #1097 worse, but @nthiery will add a check in nbgrader fetch that checks whether the submitting UID and the student id are the same or not, and throw an error if there is a mismatch (with an option to suppress the error in custom setups).

[nthiery]

Thanks!