4.x: Add the TakeUntil(Func<T, bool>) operator

[akarnokd]

Add the TakeUntil operator overload with a predicate that will complete the sequence after an item has been emitted and then the predicate returns true for it:

Observable.Range(1, 10)
   .TakeUntil(item => item == 5)
   .Subscribe(Console.WriteLine);

which prints 1 through 5 and completes.

There is a similar operator: TakeWhile which performs the check before the item is emitted.

This is one of the operators present in RxJava and mentioned in #489.

[danielcweber]

This is highly useful, we should just make sure the difference from TakeUntil(p) to TakeWhile(!p) is very clear.

[akarnokd]

Beyond the XML doc, I've tried to indicate with the stopPredicate naming that it is true that stops whereas in TakeWhile, false stops.