Use zc.buildout 3.2.0 to optionally extend local.cfg. [6.1]

[mauritsvanrees]

I added this option in https://github.com/buildout/buildout/pull/665

For inspiration, here is part of my own local.cfg:

[buildout]
develop +=
    /Users/maurits/community/zest.releaser
auto-checkout +=
    plone.releaser
custom-eggs +=
    pdbpp

allow-picked-versions = true

[remotes]
plone = git@github.com:plone
collective = git@github.com:collective
zope = git@github.com:zopefoundation

[versions]
zest.releaser =

Originally the file started with these lines:

[buildout]
extends =
   buildout.cfg

and then I always ran bin/buildout -c local.cfg. When I then wanted to use plips/plip-distributions.cfg, it got tricky because this was in a different directory, so I needed actual changes in git. Now it works in both cases thanks to the optional-extends.

[mauritsvanrees]

@jenkins-plone-org please run jobs

[mauritsvanrees]

@jenkins-plone-org please run jobs