feat(main): add prefix flag

The prefix flag lets you prefix a directory in front of the third party 
dependencies managed by goven.

Example usage:

```
$ goven -copy -rewrite -prefix="third_party" code.google.com/p/go.crypto/ssh
$ head ssh.go package main

import (
"log"
"net"
"os"


"github.com/philips/go-ssh-example/third_party/code.google.com/p/go.crypto/ssh"
)
$ ls third_party/code.google.com/p/go.crypto ssh
```