Add Standardized Webpack Configuration

As discussed in #61, WebPack is the best-in-class tool of choice and is particularly useful for React, ES6, and Gutenberg block building. It would therefore be useful to have a default, abstracted configuration for WebPack builds that a developer can pluck and start in on for their next site.

Question: Should we also maybe include the simplified version of the React WP Scripts that K. Adam has ripped out? We could also bundle this in another repo on its own.