Add additional help formatters

lib/help/added_formatters.js - HelpFormatter subclasses to
- include defaults in the argument help lines
- display descriptions in raw form (without reformatting)
- display all text in raw form

The ArgumentDefaultsHelpFormatter is probably the most useful.

lib/argparse.js - exports these new classes.

test/formatter.js - mocha tests, taken from test_argparse.py
    tests clips from the parser.formatHelp() text.
    It includes a test for array metavars.

examples/testformatters.js - same examples, but displays the help text.

formatter.js - _fillText, change forEach to map to correctly indent.
    mainly affects group descriptions.