stbt batch report: Ignore invalid "extra-columns" lines

These lines can be added by the user's classify or pre/post-run hooks,
so we should be robust to garbage.
stbt batch: summary report: List pass/fail/error counts per testcase

The code is ugly, but it works. I had initially tried to use
underscore.js's `_.groupBy` but I can't use underscore.js because of
https://github.com/asimihsan/tablequeryjs/issues/20
stbt batch: summary report: Hide subtotals by default; click to show