This closely follows the conflict handler code in argparse.py, implementing
the 'error' and 'resolve' alternatives as ActionContainer functions.

Most of the changes are in action_container.js. Changes in group.js and
argument_parser.js correctly pass the conflictHandler option to the container
and container groups (such as parser._optionals).

test/conflict.js has tests for this change, including the relevant ones
from test_argparse.py. I left one conflict handler test in base.js,
but removed the 'resolve' test. Most of the tests look at the formatHelp
output.

argument_parser - group the 4 options that the container needs first
  then call ActionContainer constructor

action_container - _removeAction - correct splice(index, 1)
group container - similar correction to _removeAction