This is an attempt at solving #44.

Syntax is as follows:

iex(1)> Poison.decode ~s({"fooBar": 1}), key_transformer: Poison.SnakeCase
{:ok, %{"foo_bar" => 1}}

iex(2)> Poison.encode %{foo_bar: 1}, key_transformer: Poison.CamelCase     
{:ok, "{\"fooBar\":1}"}

This probably kills performance, and needs some macros to fix it. However it's here as a working proposal for further discussion.

It's also likely that modifying Poison to do this is not as useful as making it a transformation step in the process. Similar to the comments in #55 this might be better done in a separate library.

Fixes #44.

Paired with @Bockit