Set up gulp to allow for dev and production environements

[briancwald]

To increase performance we should be concatenating and minify our CSS/JS for the theme. Using a tool like Gulp Util will allow us to set a --production flag which we can run various tasks:

• Concat all top level JS files in /js/ folder into a single file.
• Minify the JS
• Remove the sourcemaps from the material_admin.css
• Minify material_admin.css

Preliminary manual tests show that it's about a 600ms increase in page load.

Problem:

If we minify the js/css and set the libraries.yml to point to the production files, how can we easily switch between that version and the development version locally without manually editing the .yml file?