Exclusive usage

Looking at some tests in test_argparse.py (not included in the previous pull request), I realized that a mutually exclusive group should look like `[--bar BAR | --foo]` in the usage line. This branch corrects formatter.js to accomplish that.

Relevant tests have been added to testgroups.js.  These are a subset of the exclusive group tests in test_argparse.py

argument_parser.js changes a action.default to action.defaultValue.