blackbird style attach function for promises

[sirherrbatka]

So, blackbird has very useful (though uncommon) feature called attach: http://orthecreedence.github.io/blackbird/#attach It is nice because it bridges two distinctive approaches into one api, so we can write code like this:

handling event in the event loop
launching asynchronus task with lparallel, future is returned, we can't block event loop so we are just attaching closure that will post response back to the event loop
handling response on the event loop

without creating annoyence for simpler use cases