Upgrade Babel to 7.0.0-rc.3

[hedgepigdaniel]

This is a superset of #23

Changes

• Lift babel config into the root repository
• Use babel.config.js which supports javascript and doesn't implicitly extend .babelrc from parent directories. Config is much simpler with Babel 7's new and improved preset merging strategy. Also no need to pass { modules: false } from webpack anymore due to Make preset-env skip CommonJS compilation by default, so that Webpack itself can handle it. babel/babel-loader#660 🎉
• Update babel-loader, babel-eslint to work with it
• Fork babel-watch and merge a few changes from open PRs, and also add my own fix for a busy loop after sending a SIGINT
• Add babel 7 bridge so that jest can use babel 7 despite not having updated its dependency to the scoped package