Marching towards full support of config files: thoroughly overhauled the

command-line parsing code, splitting it up into several methods (new
methods: '_parse_command_opts()', '_show_help()') and making it put options
into the 'command_options' dictionary rather than instantiating command
objects and putting them there.

Lots of other little changes:
  * merged 'find_command_class()' and 'create_command_obj()' and
    called the result 'get_command_class()'
  * renamed 'find_command_obj()' to 'get_command_obj()', and added
    command object creation and maintenance of the command object cache to
    its responsibilities (taken over from 'create_command_obj()')
  * parse config files one-at-a-time, so we can keep track of the
    filename for later error reporting
  * tweaked some help messages
  * fixed up many obsolete comments and docstrings