Limiting to just the file(s) you're interested can make doctest start
much quicker. With a big caveat: you can limit the starting files,
but it will also test all other local files those import.
--slow turns off doctest's --fast flag, which skips reloading between
tests.
--verbose shows progress output as tests are run, if doctest 0.16.0+
is installed (and I believe will be harmless otherwise)
