Add a command for creating a grades file

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,       ,      ,     
```