Custom keybindings for custom commands

this will require https://github.com/jesseduffield/lazydocker/issues/48 to be implemented before we can go ahead, but the idea would be to allow the user to set keybindings that refer to custom commands they themselves have defined.

We currently have a 'sh' command as a custom command (partly to show the user how to make a custom command in the first place, but a user may want to swap out `sh` for `bash` etc). We could give this a custom keybinding as well, which would also serve as a good exemplar for custom keybindings on custom commands in general.