Nested raw json for encoder

Hi there!
it's very useful to be able to inform the encoder that some part of the input is already JSON.
So it should be used "as is" without any additional escaping etc.
Something like:

``` erlang
Payload = jiffy:encode(#{foo => bar}),
%%...
%%... a few hours later in another process on another node :)...
%%...
Packet = jiffy:encode(#{id => 1, payload => {json, Payload}}).
```
