feat(api): publish package with Flow annotations built in

BREAKING CHANGE: The Draft.js filters now come with [Flow](https://flow.org/) annotations. For projects that do not use Flow this has no impact. For Flow users, please make sure you correctly type your code so Flow doesn't raise issues because of conflicts with this package's type definitions.

rollup.config.js

@@ -1,13 +1,16 @@
 import babel from "rollup-plugin-babel"
 import pkg from "./package.json"
 
+const BANNER = `// @flow
+/*:: import type { ContentState } from "draft-js"*/`
+
 export default [
   {
     input: "src/lib/index.js",
     external: ["draft-js"],
     output: [
-      { file: pkg.main, format: "cjs" },
-      { file: pkg.module, format: "es" },
+      { file: pkg.main, format: "cjs", banner: BANNER },
+      { file: pkg.module, format: "es", banner: BANNER },
     ],
     plugins: [
       babel({
@@ -20,8 +23,8 @@ export default [
               modules: false,
             },
           ],
-          "flow",
         ],
+        plugins: ["transform-flow-comments"],
       }),
     ],
   },