add strtobool() (#150)

* add strtobool()

Based on the now-deprecated distutils.util.strtobool.

* add quotes

* only use default if val is None

* update function signature

Co-authored-by: Alex Fikl <alexfikl@gmail.com>

* clarify messages

Co-authored-by: Alex Fikl <alexfikl@gmail.com>