ties to
develop plugins with application or framework-specific metadata included in
the plugins' EGG-INFO directory, for easy access via the ``pkg_resources``
metadata API.  The easiest way to allow this is to create an extension
to be used from the plugin projects' setup scripts (via ``setup_requires``)
that defines a new setup keyword, and then uses that data to write an EGG-INFO
file when the ``egg_info`` command is run.

The ``egg_info`` command looks for extension points in an ``egg_info.writers``
group, and calls them to write the files.  Here's a simple example of an
extension defining a setup argument ``foo_bar``, which is a list of
lines that will be written to ``foo_bar.txt`` in the EGG-INFO directory of any
project that uses the argument:

.. code-block:: ini

    # setup.cfg
    ...
    [options.entry_points]
    distutils.setup_keywords =
        foo_bar = setuptools.dist:assert_string_list
    egg_info.writers =
        foo_bar.txt = setuptools.command.egg_info:write_arg

This simple example makes use of two utility func