Add links based on webpackPreload/webpackPrefetch magic comments

[gcoombe]

As of 4.6.0 webpack will generate links based on webpackPreload/webpackPrefetch magic comments. However where we are attempting to preload/prefetch chunks from an entrypoint loading will not start until the parent as finished (See webpack/webpack#8342 (comment))

It would be great if chunks preloaded from an entrypoint were added as link tags by this plugin. I was thinking of adding an additional includeOption like initialAndChildren.

We would need to use data exposed in stats.entrypoints[].childAssets. The use of the stats api can slow down builds but only calling getStats if the consumer opts in through includeOption: initialAndChildren seems reasonable to me.

Happy to put up a PR if this is acceptable.

[gcoombe]

Put together a quick PR #109, would be great to get some initial thoughts!