Fixed extraneous files appearing in the html-to-plaintext tarball

- this is missing from the package template, which I'll fix

ghost/html-to-plaintext/package.json

@@ -5,6 +5,10 @@
   "author": "Ghost Foundation",
   "private": true,
   "main": "index.js",
+  "files": [
+    "index.js",
+    "lib"
+  ],
   "scripts": {
     "dev": "echo \"Implement me!\"",
     "test": "NODE_ENV=testing c8 --all --check-coverage --reporter text --reporter cobertura mocha './test/**/*.test.js'",