HtmlWebpackPlugin: include main chunk

This worked perfectly fine before separating the main thread build from the workers.

Example:

new HtmlWebpackPlugin({
    chunks  : ['main'],
    // ...
});

The index files need to get created in the examples or app build scripts.

These scripts are no longer aware of the main thread chunk.

I see 2 options:

1. Manually adding the main chunk into each index file (adjusting the templates => index.ejs)
2. Evaluating where webpack stores the chunk name to file mappings and manually dropping main in there (has to happen after the builds are done).