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 


Would take the `converter` as an optional second argument and fall back to a `Ruby19.default_charset_encoder` if it isn't provided. Aim to let people pass a charset encoder when parsing a message so they can choose between exact, best-effort, charlock-sniffer, etc depending on the usage.