filter method

[rpominov]

The idea is that task.filter(predicate) will complete only when original tasks completes and the result satisfies the predicate. If the original task completes but result don't satisfies the predicate the result task will never complete.

Here is an example use case:

Task.race([
  possibleResult1.filter(isGoodResult),
  possibleResult2.filter(isGoodResult),
  possibleResult3.filter(isGoodResult),
  rejectsAfterSomeTime, // timeout
]).run(handleGoodResult, handleTimeout)