Feature Request: Add support for Vim keybinds

[cjbassi]

I think the way vtop does it is pretty great: j and k for moving in the process list along with g for going to the top and G to the bottom.

There's also a few more keybinds for other features in vtop, like dd for killing a process and h and l for zooming in and out of the graph timeline.

I also like how vtop shows a keybind help menu at the bottom of the screen. Would be nice to have that for the currect process sorting keybinds.

[cjbassi]

Just noticed there's already a PR for Vi keybinds!