Add private property to configuration

[utek]

Add private property to project properties.

Required by python-poetry/poetry#2322

[abn]

This is probably okay to go in, but I would like to wait till we have a resolution for #26 since that introduces the config section. I am not sure that is the section name we will go with, so figured it might be a good idea to do that and this together.

[sdispater]

If we go the route of tool.poetry.build (or tool.poetry.build.config) for #26, we might as well have a tool.poetry.publish (or tool.poetry.publish.config) to configure the publishing aspects of Poetry that are inherent to the project.

However, this begs the question: should it be integrated in poetry-core since the publishing part is not part of it?

[abn]

We have a bit of duplication in core and poetry. We should probably discuss how to consolidate it such that poetry code builds on top of core. But, I agree, we should introduce publish session within poetry and not core. Unless ofcourse we move the entire configuration schema to core.

[alkasm]

#26 has been merged in but this has sat stale since. Hoping to see python-poetry/poetry#2322 merged so python-poetry/poetry#1537 can be closed out. Any updates @abn @sdispater?