Add ability to mix and match webpack-webpack-plugin and html-webpack-plugin

[brunocodutra]

Fixes #136

This PR makes the configuration below valid, resulting in two separate html files, only one of which has favicons injected as described by the predicate specified in inject.

plugins: [
    new HtmlWebpackPlugin({
        template: 'a.html.tmpl',
        filename: 'a.html',
    }),
    new HtmlWebpackPlugin({
        template: 'b.html.tmpl',
        filename: 'b.html',
    }),
    new WebappWebpackPlugin({
        logo: 'logo.svg',
        inject: htmlPlugin => htmlPlugin.options.filename === 'a.html',
    }),
],