Add with_flat_map sink combinator

[exrook]

Add a with_flat_map combinator to Sink, similar to flat_map from std::Iterator

[alexcrichton]

Thanks! I wonder though, would this perhaps be better to return a stream rather than an iterator?

[exrook]

That's probably a better idea, I only used iterator because I wasn't sure how to implement it with Stream. I'll see if I can figure it out sometime this week.

[exrook]

I found the time to rewrite this patch to work with Streams rather than iterators. Let me know if you have any other suggestions.

[alexcrichton]

Thanks!