Support for html-webpack-plugin 4

Though not released yet, html-webpack-plugin 4.0.0 beta is helpful for dealing with some performance issues.

`resource-hints-webpack-plugin` silently does nothing when 4.0.0 beta is in use.

It looks like something like https://github.com/GoogleChromeLabs/preload-webpack-plugin/blob/d007cf61e575f20670d615f0cea3dc59213d2c14/src/index.js#L113-L119 is necessary to make it work.

Start by reading the linked preload-webpack-plugin implementation at src/index.js lines 113-119, then inspect how resource-hints-webpack-plugin integrates with html-webpack-plugin. Done means resource hints are emitted when html-webpack-plugin 4.0.0 beta is used instead of the plugin silently doing nothing.

Written by the indexing model from the issue text.