update README.md
implement first attempt at proper variable expansion
add common_test.py
add a coverage target, clean up lint and test targets to always run
lint common.py
undefined vars should expand to nothing
add more test skeletons
add better tests
fix rule parsing and end-of-line-continuation handling
fix pylint rule to not leave tmp files around
rename _mtime() to _stat(), handle files that do not exist as if they are infinitely old; this fixes dryrun as well
add more testing, coverage