Allow keybindings to move to the previous output

[A7R7]

The plugin oswitch only supports next_output and next_output_with_win. This is convenient enough with a 2 monitors setup.
However if you have 3 or more monitors, it is not that convenient as you have to iterate through all monitors to get to the target one, if you only use keyboard.
Thus there's a demand for prev_output and prev_output_with_win in this case.

[Kim-Dewelski]

I could try to create a plugin for this, give me a bit

[ammen99]

Duplicate of #779 (there was also a linked stale PR, feel free to take up the work, but please see the changes I requested on it)