Position window with unknown size at right bottom

[niki-on-github]

I want to place my mpv window with autofit configuration at the bottom right of my screen. Currently the move [x] [y] rule only allow to set positions by subtract fix offset values for x and y.

Since I don't know any use case where you want to move the window out of the screen to the right, I simply subtracted the size of the window when the right or bottom pane is selected with '100%-' option in my fork. Maybe we could adopt this solution or introduce a variable which is substituted by the window size at runtime for the move [x] [y] rule.