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 


Great start! Want to be able to pass a charset encoder earlier, when messages are parsed, rather than rely on a global config. Let's see your take on a `BestEffortPossiblyLossyCharsetEncoder`, too :grin: