allow setting a custom converter

I'm mostly working on inbound mail and I need to do a lot of guessing / non-perfect conversions that are a little to buggy for to integrate here, having this api would allow me to throw away a bunch of monkeypatches.

@bf4 @jeremy @mikel 


I want to do `class CustomConverter < SimpleConverter`... otherwise I have to delegate ... either way is fine if you feels strongly about the class/module thing ...