Optionals in Arrays

This PR (thanks to @jgrandelli) adds the ability to decode arrays of optionals from MarshaledObject. It also adds an optional argument to discard errors when decoding an array of objects, so that in an array where a few of the objects are malformed you don't have to throw away the whole array. You could previously do this on your own, but required an understanding of flatMap and was less obvious to new users. This API should help Marshal be more friendly.