Optimize PNG source sets

[messismore]

Is your feature request related to a problem? Please describe.
PNG sourcesets created by responsive loader are currently not optimised. The files are up to four times the size of those handled by img-loader.

This seems to be the case with both the jimp and sharp libraries that responsive loader uses to scale images. JPGs are fine, apparently.
responsive-loader/issues/63
responsive-loader/issues/3
To mitigate this it has been suggested to optimise the files after scaling.

Describe the solution you'd like
Have Webpack optimise PNGs created by responsive-loader.

Describe alternatives you've considered
From what I understand it should be possible to extend the Webpack configuration in nuxt.config.js but I am too much of a novice to get it to work. It would benefit all users of this module if this worked out of the box.

Additional context
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[archieDeveloper]

Faced the same problem. My png file, after decreasing in resolution with responsive-loader, is increased in bytes by 3 times. Paradox. I also want to process pngquant after the responsive-loader.