Feature request: Axis label spaning only a subset of subplots

[Kexikus]

This is a feature request as I have been unable to find a working solution right now. In short, I would like to see the option of label spaning for only some rows/columns.
For example, I have a plot with three rows and I would like the y label to span the first two rows (as they have the same y axis) but keep a different/unique y label for the last row.

And the same would go for x labels as well of course.

[lukelbd]

Makes sense, esp. in the context of #200. We should be able to both restrict spanning labels and shared labels to specific row/column groups with a few subplots() keyword arguments.