Make ellipsis less greedy

Make the ellipsis ("...") less greedy so arguments following the ellipsis can have their
values matched too. Allows a script like this to work as expected:

    __doc__ = """
    Usage: mycopy <src>... <dest>
    """
    from docopt import docopt
    options = docopt(__doc__)
    print options

Prior this patch docopt would greedily consume all arguments and assign them to <src>.