Add 'sampleWith' operator

[j-mueller]

The sample function currently drops the value of the notifier. It would be nice to be able to keep the notifier value like this:

sampleWith<T, A>(notifier: Observable<A>): OperatorFunction<T, Cons<T, A>>

We could then write the old sample (ignoring the notifier value) in terms of sampleWith.

[voliva]

Wouldn't withLatestFrom help?

source |> sampleWith(notifier) is almost the same as notifier |> withLatestFrom(source).

The only difference is that two emissions of notifier will result in two emissions even if source didn't change...

But then, wouldn't source |> sample(notifier) |> withLatestFrom(notifier) do the job?