Add `Util.takeWhile`.
Improve `unfold` methods.
Improve README.
Add `feedback` method and its internal implementation.

This can be directly used as a leftover mechanism.
Make `>>` into right-associative `>>:`.

Make `<<` into `:<<` just to make it symmetric.
Merge branch 'master' into leftovers

Conflicts:
	src/main/scala/conduit/IO.scala
Add a type alias for pipes with feedback.