More strict default severity, handle some Core-specific exceptions,
link to PerlTidy configuration for more accurate determination whether
the code is tidy or not, configure colours of output. Note that the
purpose of setting default severity to 1 is merely to err on the side of
caution in the event of severity not having been explicitly requested,
the guideline as adopted by the Core team in December 2018 remains
"level 4 for modified old code, level 3 for new code".
housekeeping_perlCritic.t is not affected either because it explictly
requests severity 5.