Reorganize webpack configuration structure

Created by: TheLarkInn

This PR Kicks off the process of reorganizing the current webpack configuration for CRA. We at webpack want to give first time webpack user's the best possible experience. This demographic includes developers who would eject CRA to experiment, learn, extend possibly for the first time.

We think the current config format has a lot of opportunities for organization. That being said:

Changes in this PR

• Create development and production config containing their environment specific configuration details.
• Separate module from the base webpack config and isolate to a separate file for manageability.
• Add webpack-merge as a dependency to safely compose prod and dev configs with base config
• Remove some defaulted configuration values already set for specific env's.
• Add webpack.ProgressPlugin() for development environment
• Remove chunks:'all' optimization which is only relevant for Multipage App Architectures
• Any time a CRA Issue was mentioned in comments, I removed prepended commentary (for the sake of readability.)