proposal: new include keyword in config.yaml

[severo]

A config.yaml file generally contains various layouts (eg. A4/A3/Letter, both portrait and landscape). Their structure is quite similar, and a change in one layout is generally reported in all other layouts, possibly with scale adaptation.

I think it would be easier to manage a set of layouts if each of them was defined in its own file, and then included in the config.yaml using a new keyword; such as include: A4_landscape.yaml for example.

This would allow easier management for a set of layouts, as we could copy, diff, merge them easily.

The new keyword should obviously not be restricted to layouts, but should allow inclusion of any yaml configuration subfile.