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 


@grosser Where do you think someone would need docs? They're having issues parsing emails and getting `ascii-8bit` strings, but want `utf-8`—where do they look? What do they need to understand about message charsets and Ruby string encodings to make a decision?