`Alt-s` problematic with some keyboard setups

Jetzt is great extension, but to use it without problems with my current setup (Polish keyboard) I had to change default keybinding to `Alt+j`
on this line.

> if (!instructions && ev.altKey && ev.keyCode === 83) {
> // more code
> }

It would be great if can change the default keybinding without editing source code.
If you don't mind about it, I can even write something like that.

Start at the keyboard event handler shown in the issue and inspect how the extension currently defines and applies its keybinding. Determine where a user-configurable default could be exposed without editing source code; done means the binding can be changed through the extension rather than by modifying that handler.

Written by the indexing model from the issue text.