[Distutils] Adding Provides-Extra as an edit to PEP 345

[Daniel Holth]

Hopefully this will be a more useful spec.

This begs for parenthesis in environment markers, something that is
easy to do by compiling markers to Python functions using the ast as
`markerlib` does.


Provides-Extra (multiple use)
:::::::::::::::::::::::::::::
A string containing the name of an optional feature. Must be a valid Python
identifier. May be used to make a dependency conditional on whether the
optional feature has been requested.
Example::
    Provides-Extra: pdf
    Requires-Dist: reportlab; extra == 'pdf'

...

Full changeset at
https://bitbucket.org/dholth/python-peps/changeset/4dc72bbf9ed2