Revert "#1166 validate user's options (#1177)"

This reverts commit 5bbd3a0a91.

README.md

@@ -117,6 +117,7 @@ require'nvim-tree'.setup {
 require'nvim-tree'.setup { -- BEGIN_DEFAULT_OPTS
   auto_reload_on_write = true,
   disable_netrw = false,
+  hide_root_folder = false,
   hijack_cursor = false,
   hijack_netrw = true,
   hijack_unnamed_buffer_when_opening = false,
@@ -129,7 +130,6 @@ require'nvim-tree'.setup { -- BEGIN_DEFAULT_OPTS
   view = {
     width = 30,
     height = 30,
-    hide_root_folder = false,
     side = "left",
     preserve_window_proportions = false,
     number = false,
@@ -166,7 +166,7 @@ require'nvim-tree'.setup { -- BEGIN_DEFAULT_OPTS
   },
   ignore_ft_on_setup = {},
   system_open = {
-    cmd = "",
+    cmd = nil,
     args = {},
   },
   diagnostics = {