server: cors-middleware

[odeke-em]

Fixes #7

Now sends back CORS headers for responses.

• Before

  
  $ curl -i http://localhost:9778/foo
  HTTP/1.1 404 Not Found
  Content-Type: text/plain; charset=utf-8
  X-Content-Type-Options: nosniff
  Date: Thu, 12 Apr 2018 23:31:49 GMT
  Content-Length: 19

404 page not found

* After
```shell
$ curl -i http://localhost:9778/foo
HTTP/1.1 404 Not Found
Access-Control-Allow-Headers: *
Access-Control-Allow-Methods: *
Access-Control-Allow-Origin: *
Content-Type: text/plain; charset=utf-8
X-Content-Type-Options: nosniff
Date: Thu, 12 Apr 2018 23:30:43 GMT
Content-Length: 19

404 page not found

[odeke-em]

/cc @hvent90