Add a mode where the reducer automatically does coverage-based reduction

[agroce]

Do what's in the pending blog post and the script here: https://github.com/agroce/testfs/blob/master/checkCov.py -- except make it seamless, command-line arg to the reducer, with an informative error about not finding the right clang tools if user isn't on something like our docker env. Add modes to preserve either exact coverage (hit count) or just binary (the default, what the script does).