Optparse doesn't support argparse nargs='+' style arguments. This patch
adds a callback utility function that allows that idiom. The function is
taken from the page at
http://docs.python.org/2/library/optparse.html#callback-example-6-variable-arguments.
Here's an example of how to use it:
from beets.ui import vararg_callback
parser.add_option("-c", "--callback", dest="vararg_attr",
action="callback", callback=vararg_callback)
