Adopt internal API improvements from html-webpack-plugin

[bebraw]

More: https://dev.to/jantimon/html-webpack-plugin-4-has-been-released-125d .

Maybe it's not the worst idea to drop support for webpack 3 or less and do the API tweaks highlighted in the post.

[sapegin]

Dropping webpack 3 support is definitely a good idea! Though I don't see that slow call they mention in our plugin ;-)

[bebraw]

Let's close as we don't have the slow call anyhow.