Return underlying server after calling listen

[geelen]

This is so you can call:

const pushstate = require('pushstate-server');
const server = pushstate.start({
  port: 2999,
  directories: ['./build']
});
/* some time later */
server.close()

Useful for something I'm building. Otherwise the only way to quit the server is to send an INT (I think...)

[scottcorgan]

Great! Released as 1.12.0

[geelen]

Now that's a quick turnaround! 😄