Provide functionality Cascade::hasLogger()

[DBX12]

Hi,
I would like to have a function to find out if Cascade already has a logger with a given name or not. Basically just forwarding the function of Registry::hasLogger() through Cascade::hasLogger().
If you would like that, I can open a PR, I already forked the project to provide this functionality for myself but would happily provide it to the upstream.
Cheers,
DBX12