In our use case, we are *not* reading arbitrary input that could be
malicious. Still, because we know that what we're reading is made up of
only dictionaries and lists and not arbitrary Python objects, we might
as well use the more restrictive `safe_load` function rather than the
`load` function.
Add support for YAML format configuration files. In addition, this
commit adds instructions about YAML config files to the README, and it
also changes the README to encourage use of YAML instead of JSON.
