+str - adds ZipWith and makes Zip use that to create tuples/pairs

@patriknw @ktoso @2m @bantonsson, do we want this one? Or is Flexi replacing all the things?
For some reason: no :(

"[error] /Users/viktorklang/Documents/workspace/akka/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphZipWithSpec.scala:21: missing parameter type for expanded function ((x$3, x$4) => x$3.$plus(x$4))
[error]         val zip = ZipWith[Int, Int, Int](_ + _)"