Support for a workbox.config.js override file

Created by: jeffposnick

As per https://github.com/facebook/create-react-app/pull/5111#issuecomment-425458687, there was discussion around the c-r-a v2 timeframe for allowing developers to override the default workbox-webpack-plugin (i.e. service worker generation) configuration with their own external workbox.config.js file. This functionality did not end up making it into the final release, though.

I'd love to see that happen, as putting together a one-size-fits-all default config can leave some developers confused.

I'm happy to submit a PR for that and update the docs accordingly, but I wanted to see if there was any prior art around external, user-controllable configuration files that did make it into the c-r-a v2 release. I'd rather just use a similar approach for Workbox if that solution for another plugin already exists.

If there is no prior art for configuring other plugins in v2, I'd like to confirm that folks like @gaearon and @Timer are fully on board with the idea of external config files, since it's been a contentious point in the past.