[MLton] mlb support

[Matthew Fluet]

> > If we provide enable for warn-match, then I would want it for other
> > annotations, like warn-unused and sequence-unit.  Maybe that is o.k.
>
> Maybe.  Although it's lots of flags, it does seem nice to be able to
> have a master control on the command line.  Perhaps we could use two
> flags with lots of options?
>
> 	-enable-ann {warnMatch|warnUnunsed|sequenceUnit}
> 	-disable-ann {warnMatch|warnUnunsed|sequenceUnit}

Why would we need both -enable and -disable?  By -enable, you don't mean
to force the option, overriding an annotation?