Show the which-key floating window where the current cursor is

[liancheng]

I really enjoy which-key so far, but one itchy point of mine is that, on big screens, I need to move my eyes to locate the which key window. It would be great if which-key could appear like a context menu beside the cursor. Can we allow col, row, width, and height fields in opts to accept functions, so that we can compute the location an size of the pop-up window using the current cursor position and window size?