Path prefix
I have an api that I have mounted on an url like this:
`http://localhost/v2/social/`

My provider callbacks are configured like this:
`/v2/social/connect/twitter/done`

The callbacks in between however, do not know about the `/v2/social` part. These callbacks will come back to `http://localhost/connect/twitter/callback`.

I tried setting the `grant.server.host` to `localhost/v2/social`, but this didn't work. I dived a bit into the source code and added a quick fix to  be able to specify  `path` prefix options. I tried it with express and it seems to work. 

Here is what I did: https://github.com/arjanfrans/grant/commit/b18df7033fa8756ef8e18dadc8bcc0c9df880df1

In the config I then have something like this:

```
{
  server: {
    protocol: 'http',
    host: 'localhost',
    transport: 'session',
    path: '/v2/social'
  },
  twitter: {
    key: 'keyhere',
    secret: 'secrethere'
    callback: '/connect/twitter/done'
  }
}
```

To me it seems like a missing feature. It would be nice to see it integrated. I could work on a proper pull request if I have some time next week.

Thoughts?

Thanks.
