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.
> We can get away with just the name and assume that the user will provide the actual package later.

Could you explain why we can assume that? Isn't is possible to create and use custom dgeni packages without relying on https://github.com/angular/dgeni-packages? Or should every dgeni package inherit/extend/decorate/whatever at least `dgeni-packages/base` for some common functionality like logging?