Enable minification on optimizer-express/demo/proxy.js

Hello, I need some help on enabling minification on https://github.com/ampproject/amp-toolbox/blob/master/packages/optimizer-express/demo/proxy.js
I tried to use html-minifier with a customTransformer, Middleware, but can't make it works.

I want to use this options from html-minifier:
```
const minify = require('html-minifier').minify;
minify(content, {
collapseWhitespace: true,
minifyCSS: true,
removeComments: true,
});
```
I also tried https://www.npmjs.com/package/express-minify-html as suggested by @torch2424 on slack

Start with optimizer-express/demo/proxy.js and inspect how the demo proxy handles HTML responses. Compare that flow with the html-minifier options and express-minify-html approach described in the issue; done means the proxy applies the requested whitespace, CSS, and comment minification to served HTML without breaking responses.

Written by the indexing model from the issue text.