Add index.d.ts type definitions. (#18)

Author: thorwebdev

package.json

@@ -13,7 +13,7 @@
     "test:watch": "npm test -- --watch",
     "test:examples": "node examples/",
     "cover": "nyc --check-coverage && npm test",
-    "build": "BABEL_ENV=production babel src --out-dir lib",
+    "build": "BABEL_ENV=production babel src --out-dir lib --copy-files",
     "build:umd": "BABEL_ENV=production babel --plugins @babel/plugin-transform-modules-umd src --out-dir umd_temp && webpack",
     "prepublish": "npm run clean && npm run build && npm run build:umd",
     "deploy:minor": "npm version minor && npm publish --access=public",