Added webpack watch option support.

[adaltavo]

I had the necesity to work on a development environment without using webpackDevServer and using a direct implementation of my app from another backend.

Webpack watch option was the best solution for this, and I was concerned that It wasn't avialable (yet) in create-react-app project.

With this branch now just simply run "npm run watch" for webpack to listen for changes and generate a bundle.js file under build/static/js folder.

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[gaearon]

Thanks for the PR! I don't think we'll be adding a third config at this point, but this issue is on our radar and we hope to get to it in the future.

It is tracked in #1070.

[adaltavo]

Thanks for you replay!, I know It can make the things more confusing, but for me was the easiest solution, and also required, since .dev has specific configuration for webpackdev server making the app to log many errors on console, and .prod generates a file with a unique hash everytime is compiled, and also it minifies the code, making compiling times so long. I hope you really add this feature in the future, for me (and as I can see, for many others) is a really important missing feature since sometimes (so often in my opinion) you need your app to be tested outside from webpackdevserver environment, and since it doesn't and can't generate a bundle, this is the easiest approach to solve this problem.

[AttitudeL]

Thank you so much for this! It saved my day! I am facing an issue that I am booting up the frontend code by spring-boot. Even though I enabled "proxy" setting from the package.json file, it won't bypass the oauth redirect CORS. So I need some way to constantly watching the frontend code from /target/classes/static directory where spring-boot serves the frontend code. With just a little effort, your solution worked as charm to solve my problem and now I'm able to develop the frontend so much faster without having to either acquiring session id by hacking my oauth server or recompile the entire frontend code whenever I made a change.

Thank you again for your great work and hopefully they will include this feature and provide a way to specify target directory soon.