[REQUEST] Add nice display and controls to increase or decrease it

[SteindelSE]

I know btop isn't htop, but one of the nice things that htop offers is to change the nice value of a process with hotkeys. Bog-standard top also offers displaying the nice values. For your consideration, I suggest adding NI to the proc tab, as well as keys to increase and decrease that niceness value.

Thank you for this wonderful application.