Option to listen at ipv4 only

Currently even if I specify 
```go
server := &ssh.Server{
		Addr:        "0.0.0.0:2222"
	}
```
Server will listen at both `ipv4` and `ipv6`. Because line  [server.go#L330](https://github.com/gliderlabs/ssh/blob/master/server.go#L330) has protocol `tcp`.

It would be nice to have an option to set `tcp4` instead. 