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.
> Actually I just realised that the log service is not even defined in base it is defined by Dgeni itself. So we don't need to depend upon any package here.

That sounds good! :)