BehaviorSubject counterpart?

Thank you for making this library! I'm coming from RxJS and have been looking for a lighter-weight alternative in the Python world - I'm hoping there is continuing development planned!

I was wondering if there was any planned or current counterpart to the RxPY/RxJS [BehaviorSubject](https://rxpy.readthedocs.io/en/latest/reference_subject.html#reactivex.subject.BehaviorSubject). If not, I was thinking about subclassing `AsyncSubject` to add this behavior. I'd be happy to make a PR if I get it working and it's something you'd like to have as part of this library.