[emacs] .dir-locals.el: add setup for js-mode

- move jshint option from gruntfile to .jshintrc
- remove trailing-whitespace from gruntfile and
- add jshint esversion: 6
- .dir-locals.el add locals for js-mode to use JSHint from the simple theme

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/static/themes/simple/.jshintrc

@@ -0,0 +1,8 @@
+{
+  "proto": true,
+  "globals": {
+    "browser": true,
+    "jQuery": false,
+    "devel": true
+  }
+}