Add support for selecting the nth window in a stack

[jcelaya775]

This allows you to specify a window in a stack via its index (1 through 9) in a stack. For example yabai -m window --focus stack.third.

[kiprasmel]

why overcomplicate the code with verbs second, third, etc, when could use a number instead, i.e. stack.2, stack.3, etc? would allow arbitrary amount of numbers too.

[koekeishiya]

implemented on master 84a41c9 #2342