Convert from Flow to Typescript (#816)

* convert to typescript
* add docs for custom node typing
* update webpack, gulpfile

tsconfig.json

@@ -0,0 +1,26 @@
+{
+  "compilerOptions": {
+    "target": "es6",
+    "lib": [
+      "es2017"
+    ],
+    "outDir": "dist/npm",
+    "rootDir": "src",
+    "module": "commonjs",
+    "moduleResolution": "node",
+    "strictNullChecks": false,
+    "noImplicitAny": false,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "removeComments": false,
+    "preserveConstEnums": false,
+    "suppressImplicitAnyIndexErrors": false,
+    "declaration": false,
+    "sourceMap": true,
+    "skipLibCheck": true
+  },
+  "include": [
+    "custom_typings/**/*.ts",
+    "src/**/*.ts"
+  ]
+}