Display defaults in argument help messages (#64)

The origins are here: https://stackoverflow.com/questions/23567393/python-s-argh-library-preserve-docstring-formatting-in-help-message

Thanks to:

* jdoppler @ GitHub: described the problem and possible solutions in the issue tracker.
* nilfisque @ StackOverflow: created the question at StackOverflow and summarized the
  solutions.
* @hpaulj (Paul Jacobson): provided the original solution.
* cblake (Chuck Blake): provided an alternative description and solution which
  encouraged a bit of further research.