feat: add minification for the cli and webpack-plugin

[wood1986]

R: @jeffposnick @philipwalton

fix #900

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[jeffposnick]

Thanks for the contribution, @wood1986!

We're in the process of rewriting the workbox-webpack-plugin along the lines of the proposal outlined at #1854 (Work on this has actually begun!)

Given that, I don't think it makes sense to move forward with this PR at this time. The rewrite to the plugin will result in a compiled service worker that takes advantage of minification.

[wood1986]

@jeffposnick how does it take to release v5? if it is longer than 3 months, is it possible to accept the merge?

[jeffposnick]

There were some open questions about staffing this effort that have been resolved, and work on the workbox-webpack-plugin v5 has begun. Unfortunately, I can't offer any guarantees around timelines for the release.

I do not believe it makes sense to make fundamental changes to the plugin's behavior in v4 while that effort is underway.