Allowing the whitelist to be disabled

By default it is disabled, setting the value to the empty
string will use the built-in whitelist (same behaviour than
c14n).
Canonicalization only return most common genre
Update _setup_config

Give an empty string to obtain default whitelist, None for no whitelist
or a stringlist for a custom one.
Rename/add tests

Rewrite tests now that empty whitelist is allowed.