[Question] How to pretty print?

Hi,

how can I use Poison.encode to pretty print a map?

I have been looking at the source code and I believe this should work, but it doesn't:

```
iex(1)> Poison.encode(%{"a"=>1, "b" => 2}, [pretty: true])    
{:ok, "{\"b\":2,\"a\":1}"}
```

Thank you!
