feat(i3): add keybindings to control vol/bright
feat(i3): add keybindings to control vol/bright

I've needed this for a while, just got around to doing it now.

UIsing `lib.mkOptionDefault` adds these keybinding, preventing us from
having to redefine all the bindings.

KyleOndy/dotfiles,KyleOndy/dotfiles,KyleOndy/dotfiles