nvim: Add which-key.nvim key chord visualization

Finally added a which-key like extension.
This one is a lua implementation of the old Emacs idea, but comes with
pretty sane defaults and seems less difficult to set up.

So much so that, even out of the box, it seems somewhat useful by
containing explanations of default vim bindings and showing the target
for all mark jumps. (e.g. `g`` or `g'`).

nvim/.config/nvim/lua/plugins.lua

@@ -150,6 +150,10 @@ require("packer").startup(function()
         event = "BufWinEnter",
         config = function() require('plug._toggleterm') end
     }
+    use {
+        "folke/which-key.nvim",
+        config = function() require("which-key").setup {} end
+    }
 
     -- fuzzy matching
     use {