add flipper-plugin keyword to package.json

Summary: For plugins to be discoverable through NPM, they need to have `flipper-plugin` as a keyword. This adds this keyword to all existing plugins (although they are currently not distributed via NPM) and the webside/docs.

Reviewed By: passy

Differential Revision: D17395079

fbshipit-source-id: e42dd31ca39c3754cf729052ee783687748bafe8

docs/tutorial/js-table.md

@@ -49,6 +49,7 @@ you can also specify a title to show in the Flipper sidebar and an icon to displ
   "version": "1.0.0",
   "main": "index.tsx",
   "license": "MIT",
+  "keywords": ["flipper-plugin"],
   "icon": "apps",
   "title": "Sea Mammals",
   "dependencies": {