Add convenient passthru method

[jcubic]

There is a need for the API that will just use the same request but to the server. This similar to #22 that is not enough.

Consider code of syntax highlighting:

app.get('*', async function(req, res) {
  const url = new URL(req.url);
  const extension = path.extname(url.pathname).substring(1);
  const language = language_map[extension];
  const accept = req.headers.get('Accept');
  if (language && Prism.languages[language] && accept.match(/text\/html/)) {
      const code = await fetch(req.url).then(res => res.text());
      const grammar = Prism.languages[language];
      const tokens = Prism.tokenize(code, grammar);
      const output = Prism.Token.stringify(tokens, language);
      res.html(`${style}\n<pre><code class="language-${language}">${output}</code></pre>`);
  } else {
      const fetch_res = await fetch(req.url);
      res._resolve(fetch_res);
  }
});

from gh-pages branch: sw.js.

So there is a need to use hidden _resolve method that accept response object.

[jcubic]

It seems that there is already:

res.fetch(req.url);

But it would be better to use a request object that have all the headers not only URL.

So the fetch need to be overloaded.