Add fromBind to replace direct StreamTransformer (dart-lang/stream_transform#35)

Fixes dart-lang/stream_transform#34

`new StreamTransformer(onListen)` produces a Stream which can not know
whether it is a Broadcast stream. The `bind` method can more directly
express the intent of these transformers but adding a class is a lot of
boilerplate. Add fromBind to handle these cases.