Allow alternative server by options

[ghost]

Custom http server can be passed to options.
It has check for server in options to have .createServer method or else use default

Update examples where possible for usage of server option.
Excluded https as it takes options for ssl.

Cleanup of #13

Example

import http from 'somemagic-http-server';

polka({ server: http });

[roblav96]

@lukeed This is a very much needed feature. +1 merge

[lukeed]

Sorry for the delay on this 🙈 I've had something similar locally for a little while (thanks to @ThiccCat for the initial idea).

This PR doesn't account for the different createServer signatures from https and http2 modules. They need (opts, handler) -- and I imagine some other custom servers may do the same.

The only things we actually care about wrt the server are (1) passing it the Polka handler and (2) calling listen on it.

Because of this, my change is to pass in an actual server to Polka options if we're supposed to hook into it. Otherwise, if nothing is passed, a default http server is created when listen is called.

Attaching to https may look like this:

const https = require('https');
const polka = require('polka');

const server = https.createServer({ key, cert });
const app = polka({ server }).get('...', noop);

await app.listen(PORT); // this is where Polka attaches "handler" to "server"

Similarly, for uws custom http:

const { http } = require('uws');
const polka = require('polka');

// OPTION 1 :: No changes
const { handler } = polka().get('/', noop);
http.createServer(handler).listen(PORT, err => {
  console.log(`> Running on localhost:${PORT}`);
});

// OPTION 2 :: Pass server into Polka
const server = http.createServer();
polka({ server }).get('/', noop).listen(PORT).then(() => {
  console.log(`> Running on localhost:${PORT}`);
});

Lastly, for basic use, including Polka sub-applications, there's no change at all:

// no http server here
const sub = polka().get('/', noop); 

// no http server here either
const main = polka().use('/users', sub).get('/', noop); 

await main.listen(PORT);
//=> NOW only "main" app has a server booted