Add the possibility of putting grade specs in a separate file

[slarse]

It makes sense to be able to put the grade specs in a file, such that it too can be stored in version control. The file format would simply be one line per grade spec.

This should also add a new CLI option to specify the file, --grade-specs-file, which should be mutually exclusive to --grade-specs.

If we also add a grade_specs_file option to the config file, there will also be a need to check for mutual exclusivity in the config to avoid ambiguity. You either put in grade specs inline in the config, or provide a path to the specs file.