Factored out code for extracting-or-creating one of the option

Factored out code for extracting-or-creating one of the option
  dictionaries in 'self.command_options' to 'get_option_dict()'.
Simplified code in 'parse_config_files()' and 'parse_command_line()'
  accordingly.
Fixed code in constructor that processes the 'options' dictionary
  from the setup script so it actually works: uses the new
  'self.command_options' dictionary rather than creating command
  objects and calling 'set_option()' on them.