content_log configurability and testing

This makes content_log a bit more configurable and also more easily customized via isa_plugin()

It also adds test coverage
since this is a "one time" expect, why not permit the contents of the config file to be any true(ish) value? 

if ($enabled =~ /^(true|1|yes)$/) or something like that seems friendlier to me. Adding in some POD wouldn't hurt either. :)