Allow sxhkd-like shortenings

[exic]

It could make our config files more concise to have those shortenings which sxhkd provides (see e.g. https://github.com/baskerville/bspwm/blob/master/examples/sxhkdrc).

So

cmd + alt - {1-9}: yabai -m space --focus {1-9}
cmd + {h,j,k,l}: yabai -m window --focus {west,south,north,east}

would be the same as

cmd + alt - 1: yabai -m space --focus 1
cmd + alt - 2: yabai -m space --focus 2
cmd + alt - 3: yabai -m space --focus 3
[...]
cmd + h: yabai -m window --focus west
cmd + j: yabai -m window --focus south
[...]

(previously accidentally posted in koekeishiya/dotfiles#13)