Add CLI support for --dev and --prod options

Currently stripes-core's build and serve logic always use the prod webpack config for build and the dev webpack config for serve. There are, however, times when it is handy to serve a prod configuration or build a dev one.

The -dev and -prod options are already stubbed out in the CLI help. To complete this the CLI will need to:

Default serve to -dev and default build to -prod.
Specify NODE_ENV=development for --dev when not already set.
Specify NODE_ENV=production for --prod when not already set.
Pass dev and prod options along to stripes-core.
Stripes-core then needs to be updated to pick up the appropriate webpack.config upon receiving dev or prod. This may require some work to ensure the configs are interchangeable finally closing out STCOR-29.