Added support for overriding auto-generated nicknames in route config

I thought it'd be helpful to be able to override the automatically generated nickname with one provided in the route.

For example:

```
server.route({
    method: 'GET',
    path: '/user/{username}/friends/',
    config: {
        id: 'getFriendsByUsername',
        plugins: {
          'hapi-swagger': {
            nickname: 'getFriendsByUsername'
          }
        },
        handler: function(request, reply){
          reply('ok');
        },
        description: 'Get friends for username.',
        notes: ['Returns friends for the provided username.'],
        tags: ['api'],
        validate: {
        params: {
              username: Joi.string()
                      .min(3)
                      .max(10)
                      .required()
                      .description('the first number')
          }
      }
    }
});
```