Expose logging settings through CLI and environment

[birdzix]

It would be nice to be able to configure the logging options via command line args and/or environment variables. For example, we could provide --file-log-level and --stream-log-level as command line args for configuring the logging levels.

If a setting has a command line arg defined, use that (highest priority).
If a setting has an environment var defined and no command line arg, use the env var.
If no env var is set and no command line arg is passed, use the default value.

It may be worth investigating collections.ChainMap for this.

[danielballan]

It might also be useful to look at how tornado's settings functionality works. (I don't know and can't speak to the implementation at all, but it seems to work well.)