webpack implementation improvements

[ptrckvzn]

• not using any environment variables in the npm scripts for better windows compatibility (removed SAGE_ENV)
• extracted options to a config file
• dist/ keep the same assets folder structure (images/, fonts/, styles/, scripts/) - this work better with the assets loaded via php as it expect subfolders to the dist folder
• webpack.config has settings specific to the "watch" script
• renamed the watch script to watch.js
• add image-webpack-loader for optimization

[retlehs]

thank you for your continued efforts on this 💙

[ptrckvzn]

No problem. Thank YOU guys for Sage/Trellis/Bedrock. :) I use those on every project I work on and I am currently working on a few ones using this sage branch, so I will keep improving as I go.