Add missing docs for --ignore-compile
Add perl5/parse-json using JSON::PP
Compute an overall 'score' across all tests

The score is computed using the geometric mean of relative rates
compared to a reference compiler (the first one in the timings file).
It is expressed as "% of reference compiler's overall speed", so the
reference compiler has a score of 100; higher is better, so 200 would
be twice as fast "overall" compared to the reference.