Simplify configuration options

Programs should allow in this order: cli options, user configuration files passed by cli, environment variables, system configuration files and program defaults. There seems there's not a common easy way to achieve this in Python. sbws accepts cli and user configuration files in a complicated way. The program defaults are provided as a configuration file which might not be that convenient seems confuses where to find the defaults. Maybe an external library like https://github.com/bw2/ConfigArgParse (supported in Debian) or some other design can help to simplify this.