Add option for entrpoint + preload/prefetch children

[gcoombe]

Fixes #108

Description

• Adds link tags for entrypoints as well as any chunks which are prefetched or preloaded from an entrypoint via a magic comment

Why

• If we use magic comments to preload bundles from an entrypoint, the bundle will not start loading until the parent as finished (See https://github.com/webpack/webpack/issues/8342#issuecomment-438649676)

TODO

Submitted a PR to get early feedback on the approach but would need to do the following:

• [ ] Add to README
• [ ] This requires webpack >= 4.6.0 so we would either need to bump the peer dependency or make it clear that child assets will not be added in an older verions (would prefer the former)