Feature request: wait for http over socket

I have the following pattern to wait for Docker to be running:

```bash
wait-on -s 1 -t 1000 --verbose socket:/var/run/docker.sock
wait-on http-get://unix:/var/run/docker.sock:/info
wait-on -s 1 -t 1000 --verbose http-get://unix:/var/run/docker.sock:http://latest/info
```

This makes use of the [wait-on](https://github.com/jeffbski/wait-on) utility.

It would be very handy if `wait4x` could have the ability to wait for sockets in general, and http-over-sockets in specific.