Make a new kind of template available to `layout.html.st`

[scmbradley]

I'm not sure if this is possible currently, I couldn't work out how, anyway. Basically, I'd like a way to define something in index.yaml such that I can put it somewhere in the layout.html.st file. So yst defines $nav$ and $content$ based on what it reads in index.yaml. I'd like to make it such that there could be a third column, for instance. Or a sort of headline banner that runs across both nav and content and displays a different picture on each page.

Obviously, I could make it part of $content$ and then do some CSS trickery to make it appear where I want, but it seems like a neater solution would be to just place it at the right place in the layout file.

Is this currently possible? Would implementing it be nontrivial?