Add statistics to regression.js

[johnkenny54]

I've been using regression.js a lot locally to test the Oxygen icons and W3C files, both to isolate problems and to verify that my changes aren't having any unintended side effects.

These changes should have no impact on the regressions being run as part of CI.

Changes to regression.js:

• Record statistics during test run (input file length, optimized file length).
• Write a timestamped .tsv file to tmp directory at end of run (for each file, show pass/fail, orig length, optimized length, reduction in size).
• Show total reduction in file size on console at end of tests.
• Added some configuration constants. Eventually I would like to make these command line options. I've been using this file to track down the regression errors with the Oxygen icons and it's very helpful to be able to run the regressions on a subset of files and/or with alternate configurations.

Added tmp to .gitignore

[SethFalco]

Thanks for peeking at this. I agree the output of our regression tests could be better, but I disagree with how some of this was approached.

In particular, I'd prefer if we kept regression.js as a simple CLI tool that just reads args and writes to STDOUT, and we can work on improving that experience rather than maintaining multiple output formats.

One notable change that would be nice is to drop logs for passed tests. Given the number of tests we'll have once the updated regression tests have been merged, they won't be practical anymore.