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 


As tenderlove once tweeted "I knew a guy who preferred inheritance over composition. He was a 'super' guy".

Wouldn't composition be a better pattern? The test you wrote implies replacing the converter, rather than extending it.

I don't feel strongly if there's a good use-case for inheritance.