Implement a Flow plugin for Razzle

[pugnascotia]

This PR adds a plugin that adds Flow typing. The plugin:

• Adds a webpack plugin to run Flow typing during the Webpack build
• Amends the Babel config to add the Flow preset
• Amends the ESLint config to add the Flow preset and recommended rules

To test:

1. Create a Razzle app
2. Add the plugin.
3. Start it (it should start cleanly)
4. Edit src/App.js and add the line const foo: number = 's'; e.g. after the imports
5. ESLint will complain about the unused variable but cope with the Flow syntax.
6. Flow will complain about a type mismatch

[jaredpalmer]

👋🏻 Hey! Thanks for this PR. It looks good. Unfortunately, beyond being a breaking change, I am not able to commit resources to maintaining any Flow or Flow-related code at this time. If you’d like to publish this as a standalone plugin, feel free.

[pugnascotia]

Thanks, I've done as you suggested - https://www.npmjs.com/package/razzle-plugin-flow