Forbid using ! in import paths

Created by: gaearon

I've heard a few reports that people use Webpack import extension syntax (like !!file!./stuff) to customize Create React App loaders. We don't support this officially, and we may accidentally (or on purpose) break this in the future. This also couples apps to Webpack resolution mechanism too hard.

We need to write a Webpack plugin that throws in parsing stage (https://github.com/webpack/docs/wiki/plugins#the-parser-instance-compilerparser) if you attempt to require or import something with ! in the path.