[DESIGN] dataflow for composability (#515) (#645)

* Add design doc for dataflow for composability (#515)

This proposed design doc addresses an issue titled
"inspecting formattable values", which is really about dataflow
through the formatter and structuring it to make function calls
compose with each other.

* Add pull request link

* Add link to spec PR

* Add correct image for second diagram

* Add one more alternative

* Add another alternative