[Feature] Add ability to specify [hash] in navigationFallback

[mike-marcacci]

Library Affected: workbox-sw

Browser & Platform: all browsers

Issue or Feature Request Description: I have the need to specify a navigationFallback that uses webpack filename substitutions, like this:

{
  new HtmlWebpackPlugin({
    template: "src/boltline.html",
    filename: "boltline-[hash].html"
  }),
  new GenerateSW({
    // ...
    navigateFallback: "/boltline-[hash].html",
    // ...
  })
}

Our caching strategy depends on idempotent URLs, or we risk serving old content for a long time.

[vasisouv]

This is not yet implemented, right? Same goes for the precacheFallback.fallbackURL, which can not be made dynamic at the moment.