add a test that would fail with the restricted whitelist approach

If no option is set, valid genres should not be rejected. Which is an argument
to have a wide default whitelist.
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.
Merge branch 'KraYmer-lastgenre-tests-new' into lastgenre-tests-new

Conflicts:
	test/test_lastgenre.py
lastgenre: Restore default whitelist