TestRunner: Added --chart to display a chart of performance tests

This uses the Chart.js javascript library.
Everything is put into a standalone .html file and opened with the
default browser.
I also simplified the text output to use the same data as the chart.
You can now use a commit range with git's ".." syntax.
The --ci option will use results from CI (you'll need to fetch them
first):

  $ git fetch https://gitlab.haskell.org/ghc/ghc-performance-notes.git refs/notes/perf:refs/notes/ci/perf
  $ python3 testsuite/driver/perf_notes.py --ci --chart --test-env x86_64-darwin --test-name T9630 master~500..master