add custom attribute support for preload/prefetch

[moyus]

When maintain a large web application, we need to deploy static assets on CDN, which has a different origin against our web domain. So in order to use tools like sentry to track js errors, we need to add crossorigin="anonymous" attribute on scripts.

However, when use preload/prefetch feature of script-ext-html-webpack-plugin, the output <link /> don't contain crossorigin="anonymous", which will cause double fetches.

<link ref="preload" as="script" href="//a-cdn-server.com/some-script.js" />
...
<script src="//a-cdn-server.com/some-script.js" crossorigin="anonymous"></script>

This PR is try to add a feature to support custom attributes for link tag.

example:

plugins: [
  new HtmlWebpackPlugin(),
  new ScriptExtHtmlWebpackPlugin({
      custom: {
        test: /\.js$/,
        attribute: 'crossorigin',
        value: 'anonymous'
      },
      preload: {
        test: /\.js$/,
        attrs: {
          crossorigin: 'anonymous'
        }
      }
  })
]  

[numical]

I can see this being useful.
The problem is a messy configuration API - with custom attributes in two places.
Though it will be a harder implementation we really need to look at using the existing custom config in resource hints as well.
Also I'll need some tests please!

[numical]

Hi,
I found a neater way of doing this - the custom attributes are added last so all I had to do was extend this processing to <link ... as="script" ... /> tags.
See this commit for the details.
This is available from v2.1.0 onwards.
Let me know if it does not work for you.

[moyus]

Seems great!