Add configurable sanity check routine

An example of what SANITY_CHECKS would look like is:
SANITY_CHECKS = [
    (os.path.join(PROD_DIR, 'com/mozilla/mozilla.com.public'), (
        (r'^mozilla\.com\.\s+(\d+\s+)?IN\s+A\s+'),
    )),
]
Merge branch 'master' into prod