Multiple default arguments

This is in respond to https://github.com/docopt/docopt/issues/127

"""
Usage:
    cleanup.py
    cleanup.py [--data=<data>]

Options:
    -d --data=<arg>    Input data [default: foo, bar]

"""



by default, this will return a list containing two parameter, if only one value, it will not be a list.

I added a testcase for this as well. you can check it up.