Partial export

[gforcada]

Part of #86

Thanks a lot to @cihanandac for plenty of help, code and lots of ideas!

This is a first step on being able to export a partial site:

• provide a simple list of paths to be exported, rather than export the complete site

The idea is to make it simpler/easier for integrators to decide what needs to be exported, see #86 for ideas that are being discussed.

But as things need to be proved small, what about this first approach? does it feel that is on the right direction?

[mister-roboto]

@gforcada thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!