show diagnostics on parent dirs if nodes with diagnostics are not visible (#862)

doc/nvim-tree-lua.txt

@@ -85,6 +85,7 @@ function.
       update_cwd          = false,
       diagnostics         = {
         enable = false,
+        show_on_dirs = false,
         icons = {
           hint = "",
           info = "",
@@ -231,6 +232,11 @@ Here is a list of the options available in the setup call:
       type: `boolean`
       default: `false`
 
+  - diagnostics.show_on_dirs: if the node with diagnostic is not visible,
+    then show diagnostic in the parent directory
+      type: `boolean`
+      default: `false`
+
   - |diagnostics.icons|: icons for diagnostic severity
       type: `table`
       default: `{ hint = "", info = "", warning = "", error = "" }`