Add `dvm ip` command to output the address of the Vagrant machine

This adds a helper to print the configured (or default) IP address of the Vagrant VM. Similar to [`boot2docker ip`](https://github.com/boot2docker/boot2docker#container-port-redirection), except just reads the configuration and doesn't try to connect to the machine itself.

Example use case would be to connect to published container ports, like:

```sh
dvm up
docker run -d -p 80:80 my_web_server
curl -i "http://$(dvm ip)/foo"
```