Crash if given --column and --columns at the same time
Refactor to allow command-line parsing reuse

Refactor to allow other projects to reuse losser.cli's command-line
parsing code in their own command-line parsing:

+ Add make_parser() function that returns the ArgumentParser object
  (other projects can call this to get an ArgumentParser to use as
  the parent of their own ArgumentParser object)

+ Add parse() function that does the actual parsing (including
  losser.cli's post-processing after argparse's parse_args() has
  finished) and returns the resulting argparse Namespace object.
  Other projects can call this, passing in their own ArgumentParser
  object, to do the parsing and get the result.

+ Add do() function that is a replacement for the previous parse()
  function, for tests to call.