Improve CLI: Add option to change log level

The CLI now accepts a `--log {level}`/`--l {level}` option which overrides the configured log level from other packages. This is an opt-in: if you don't use `--log {level}`/`--l {level}` nothing happens.

I think this is a nice way to improve the CLI.
Anyway... with adding CLI options it is maybe better to switch from https://github.com/substack/node-optimist to https://github.com/visionmedia/commander.js#option-parsing as it generates a nice `--help` option, too.