Initial commit: Developer Tools MVP with visual editor

- Complete React app with 7 developer tools
- JSON Tool with visual structured editor
- Serialize Tool with visual structured editor
- URL, Base64, CSV/JSON, Beautifier, Diff tools
- Responsive navigation with dropdown menu
- Dark/light mode toggle
- Mobile-responsive design with sticky header
- All tools working with copy/paste functionality

node_modules/underscore/modules/difference.js

@@ -0,0 +1,13 @@
+import restArguments from './restArguments.js';
+import flatten from './_flatten.js';
+import filter from './filter.js';
+import contains from './contains.js';
+
+// Take the difference between one array and a number of other arrays.
+// Only the elements present in just the first array will remain.
+export default restArguments(function(array, rest) {
+  rest = flatten(rest, true, true);
+  return filter(array, function(value){
+    return !contains(rest, value);
+  });
+});