For the next release, I added the numpad plus/minus (internally referred to as add/substract keys, Windows-keycode 107 and 109) as keys to zoom in/out.
I am hesitant to use the usual character keys for "+" and "-" (internally referred to as OemPlus/OemMinus, Windows-keycode 187 and 189) to zoom in/out, because it is much more likely to hit them accidentally. And I still believe that most people will find the mouse wheel easier to use, so I would like to optimize for this usage.