Support severtity level for packages/classes per writer

[dzikoysk]

I'd love to see a possibility to set the severity levels for packages or classes for each writers individually.

In our use-case we'd like to filter a little bit of logs from the console, but still keep it all in the file - unfortunately it's not possible with the current impl because the level@package property works globally and affects all writes. Ofc I can filter this using a custom writer, but I think it might be a better idea if it would work like that out of the box. In the end, in most of the use-cases if we'd like to get rid of some logs at the given level, we're mostly thinking about a specific writer that is quite annoying, not the whole logging system.