grader: Allow OK feedback to be on separate, third line

[fushar]

Previously, to give a feedback from a scorer with a custom points, one would need to follow this format:

OK
50 a feedback

where 50 is the point and a feedback is the feedback.

This format is a bit weird and is not consistent with AC / WA verdicts, so with this PR, we allow this alternative format:

OK
50
a feedback

The old format is still supported for backward compatibility.