Added the ability to pass an extra extends file.

I'd like to use Plock to make it easy to get a project of ours running. I'd like to target Python developers that don't know Plone, so pip is an obvious choice.

The project I'm talking about is https://github.com/Onegov/onegov.municipality and it's a bit of a distribution of sorts with a number of third-party packages which we integrate and pin to specific versions.

Now technically, this would work:

    plock -a onegov.municipality .

But the versions are going to be all over the place and currently it won't even install because of a version conflict.

That's why I extended plock thusly:

    plock -a onegov.municipality -e "https://raw.githubusercontent.com/OneGov/onegov.municipality/master/release/latest.cfg" .

This adds an extra extends line to the buildout.cfg, below the original line. This essentially overrides all versions in the first extends file and enables us to use Plock.

I'd love to see this feature in Plock. It's not really a big change in the code so I just went ahead and did it. Feel free to ask me for changes or to do this in a smarter way :smile:

Thanks for Plock in any case! I think there are some interesting things that could be done with it.