Add files to package.json. (#388)

Move from `.npmignore` to `files` field in `package.json`.

Author: wtgtybhertgeghgtwtg

.npmignore

@@ -6,6 +6,7 @@
   "engines": {
     "node": ">=0.12.0"
   },
+  "files": ["index.js", "locals.js", "lib"],
   "dependencies": {
     "babel-code-frame": "^6.11.0",
     "css-selector-tokenizer": "^0.7.0",