Add a command for creating a grades file

[slarse]

Something like this:

repobee grades create-grades-file --assignments task-1 task-2 task-3 --students slarse glassey glennol --grades-file grades.csv

should create a grades.csv file that looks something like this:

username,task-1,task-2,task-3
 slarse,       ,      ,     
glassey,       ,      ,     
glennol,       ,      ,     

[slarse]

@marceloFA Assigning you to this issue, let me know if you don't want to do it :)

EDIT: Actually, I can't assign you until you comment on it. But feel free to keep working on it and just drop a comment here that you've started.

[marceloFA]

Sure, assign me to this pls :)