Add a Webpack build process and initial App that loads in Gutenberg [4 hrs.]

[adamsilverstein]

Related: #2

• Add skeleton webpack configuration.
• Add skeleton app that to build with webpack.
• App should live in a src folder, build should go to a dist folder.
• Ensure built app javascript is loaded from php on the post editor when gutenberg is being used
• needs to work with WordPress 5.0 + and ideally wp4.9 with the plugin enabled (although the GB project has officially dropped 4.9 support for the plugin, so this is less of a priority if compatibility is not straightforward.