Add a method to list the current named logging levels

[Edward Spencer]

Sometimes I like to pass the logging level up to the command line params so my user can specific what level of logging they want. However there is no easy method for pulling the named logging level names.

Looking into the code, it would actually be incredibly easy to implement;

in `logging.__init__.py`;

def listLevelNames():
    return _nameToLevel.keys()

You could obviously add some other features, like listing only the defaults, sorted by numerical level or alphabetically, etc. But really this basic implementation would be enough to expose the internal variables which shouldn't be accessed because they change (and in fact, between python 2 and 3, they did).

Any thoughts?

Thanks,
Ed Spencer