Allow starting dummyserver in standalone mode

I didn't find a way to launch dummyserver stand-alone. Helps writing tests if you can launch the dummyserver and just shoot at it with HTTPie.

Any thoughts how to improve this? Is there a place where this fits on documentation? I'm happy with a doc-only fix if there's another way to run this.

```
(.env)grant:urllib3 joneskoo (dummyserver-standalone) $ python -m dummyserver.server
Listening on http://127.0.0.1:51163
```

```
grant:~ joneskoo $ http get http://127.0.0.1:51163/specific_method?method=GET
HTTP/1.1 200 OK
Content-Length: 0
Content-type: text/plain
Server: TornadoServer/3.1.1

```