Improved output for batch grading

[jathak]

targets batch currently outputs a really bad plaintext log of all tests. This is fine enough when grading only a few students at a time, but it's annoying to scale this up to a full class of students. At a bare minimum, we need to add CSV output, and possibly some other options.

Along with this, we may want to change the current targets batch command to use standard output instead of writing to log.txt.

[jathak]

This was added as part of 0.8.0 (which is technically still on the experimental branch, but that's mostly because the server needs a bit more fine tuning). targets batch --json and targets check --json use a new JSON output. The new web console will format this into a nice table, but we can add CSV output (by converting the JSON) if we need it later.