Add sprintfAddHandler and use it.

This allows you to register a handler which knows how to convert types NQP doesn't know about to bigints.  (Eventually I hope to expand it to floating point values and strings as well.)

I'm not particularly attached to the name sprintfAddHandler, if someone has a better suggestion.
Fix sprintfAddHandler return value.

I deleted the return value just before pushing the last patch.  Turns out it was important.  Whoops.
Added --no-clean and updated usage.
Change name sprintfAddHandler to sprintfaddargumenthandler.