neovim: Add hologram.nvim

For historical purposes, I am adding hologram.nvim in a commit, although
I should be able to replace this with image.nvim.

home/neovim.nix

@@ -311,6 +311,15 @@ in
         type = "lua";
         config = ''require("auto-save").setup()'';
       }
+      {
+        plugin = hologram-nvim;
+        type = "lua";
+        config = /* lua */ ''
+          require('hologram').setup {
+            auto_display = true
+          }
+        '';
+      }
       {
         plugin = toggleterm-nvim;
         type = "lua";