HAL Forms support?

I see #78, but this is more of a feature request.

Something as simple as this could be used to mark a route as a form.

``` js
server.route({
  method: 'POST',
  config: {
    handler: (req, reply) => {/*...*/},
    plugins:{
      hal:{
         form: true,
       },
    },
  },
})
```
