Add ignoreNil function about reactkit

[inamiy]

Hi @davdroman, thanks for your kind words 😊
But I'm sorry to say that I'm no longer maintaining this project because there are more awesome Swift reactive frameworks nowadays, and among those I recommend to use RAC once again for your new project (sorry this sounds ironical).
There are a number of good reasons for that, which you probably already know.
If you are still interested in using ReactKit, that's absolutely fine, and I can merge your PR :)

[davdroman]

It's sad to hear that, @inamiy :(

I'm still sticking to ReactKit for the sake of KVO sugar. I'll play around with it and PR if I find it to be a better solution for my case (which I believe it is).