custom node keymaps (#855)

lua/nvim-tree/config.lua

@@ -58,6 +58,7 @@ function M.get_icon_state()
   }
 end
 
+-- TODO: remove this once the cb property is not supported in mappings
 function M.nvim_tree_callback(callback_name)
   return string.format(":lua require'nvim-tree'.on_keypress('%s')<CR>", callback_name)
 end