Introduce support for categorizing plugins

Summary:
This PR introduces the possibility to group plugins in categories.
The category can be determined by setting the `category` field in `package.json`.
Categories are sorted alphabetically.
Categories are shown below all uncategorized items.
Within categories, items are sorted as before: by last recently usage.

Design wise, the category name might now look more prominent than the app name, this is to be addressed in a follow up PR.

Reviewed By: jknoxville

Differential Revision: D18169459

fbshipit-source-id: 77deb0f27a0462a0d449944ddc262396160687a2

docs/tutorial/js-table.md

@@ -52,6 +52,7 @@ you can also specify a title to show in the Flipper sidebar and an icon to displ
   "keywords": ["flipper-plugin"],
   "icon": "apps",
   "title": "Sea Mammals",
+  "category": "Example Plugin",
   "dependencies": {
     "flipper": "latest"
   }