#36 [ENH] Use standard dev logging

We've got a janky BaseCog.log method but it would be better to use a more standard logging mechanism. Python seems to have something for this natively. Goals: - More granularity for debug, info, warn, error severity levels - Domain constraints so that log levels for different cogs or other functional areas can be adjusted for debugging - Log levels changeable at runtime so bot doesn't need to be restarted or need code changes - Message templating so expensive string interpolation is deferred until after the log level checks have determined the message will actually be shown