Improvements to logging

[jenshnielsen]

• Avoid calling getLogger on each logging call. This avoids 2 or 3 unnecessary function calls for each log call
• Don't set the level on each individual logger to a hardcoded constant when getting the logger. This makes it hard to be able to consume the loggers since every time getLogger is called the level would be overwritten. Rather leave the level unset and rely on the level of the parent logger.
• To enable the above ensure that the level of the "plottr" logger is set to match the level of the handler attached to it when setting up a handler (both for stream an qt widget handler)
• Add a commandline argument to enable the user to toggle the console output level for easier debugging.