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 


@jeremy @mikel thoughts on this implementation? Naming? I'm not sure what the class variables buy us (as opposed to class instance variables... is this subclassed anywhere?)

An API like this should be documented (besides the test).