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
```