Registers an unpack format.
`name` is the name of the format. `extensions` is a list of extensions
corresponding to the format.
`function` is the callable that will be
used to unpack archives. The callable will receive archives to unpack.
If it's unable to handle an archive, it needs to raise a ReadError
exception.
If provided, `extra_args` is a sequence of
(name, value) tuples that will be passed as arguments to the callable.
description can be provided to describe the format, and will be returned
by the get_unpack_formats() function.