It's unfortunate (at least to me, but I know I have a skewed view of
this) that the help string in add_argument uses %-formatting when
formatting the result. I'd much rather it use str.format().

I see a couple of options:
- leave it as-is and live with %-formatting forever
- add a different, mutually exclusive parameter,
  like help_ex or help_str
- try to guess whether to use %-formatting or str.format,
  based on inspecting the help string

I'm not really wild about any of these.

I'm still looking through the code, slowly.