Set up flow config; add third party libdefs

package.json

@@ -15,6 +15,7 @@
     "build:umd": "webpack --config webpack.config.js --sort-assets-by --progress",
     "deploy:examples": "git checkout gh-pages && rm -rf ./storybook && mv dist-examples storybook && git add -A && git commit -m \"Storybook deploy\" && git push origin gh-pages && git checkout -",
     "examples": "start-storybook -p 9001 -c ./examples/.storybook --dont-track",
+    "flow": "flow",
     "fmt": "find examples performance src -name '*.js' | grep -v -E '(node_modules|dist)' | xargs npm run fmt:cmd",
     "fmt:cmd": "prettier --print-width=100 --single-quote --write",
     "lint": "npm run lint:cmd -- examples performance src",