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

src/plugins/sandbox/package.json

@@ -4,6 +4,7 @@
   "version": "1.0.0",
   "main": "index.js",
   "license": "MIT",
+  "keywords": ["flipper-plugin"],
   "gatekeeper": "sonar_sandbox_plugin",
   "icon": "translate",
   "bugs": {