Allow separate row/col fractions for zoom_pane

[stinos]

I typically use zoom_pane to get more vertical space when working on a single file. However on wider screens this wastes a lot of screen space for text files which are already limited to a suitable number of columns (think 90-110). It would be very convenient if zoom_pane would leave the horizontal size intact so it leaves 2 panes side-by side unmodified.