Add commas to output counts.

    From: http://code.activestate.com/recipes/498181

def _add_commas(s, sep=','):
    """Add commas to output counts.

    From: http://code.activestate.com/recipes/498181
    """
    if len(s) <= 3:
        return s

    return _add_commas(s[:-3], sep) + sep + s[-3:]