Allow exporting of all score tags

[jathak]

We used the correctness and private score tags for a couple of specialized scores around the midterm for 61A this summer not realizing that the grade exporter was limited to max(effort, total, regrade) and max(composition, revision).

These were all imported anyway, so I'll just take them from our original sources, but it would be nice to have some way of customizing what scores get exported, or at least some way to just export all score tags.