Handle a configuration file specified on the command line.

 * pyrseas/cmdargs.py (cmd_parser): Initialize global _cfg here.
   (parse_args): If config file specified, load it and merge it.
 * pyrseas/config.py (Config.__init__): Use new merge method.
   (Config.merge): New method to merge configuration dicts.
 * tests/test_config.py: New test.