Watch macro files in cljs.build.api/watch

The cljs.build.api/watch function is great for writing scripts or tooling but it doesn't track .clj macro files as noted in here. Direct users face unexpected behavior and tooling developers need to reimplement the watch functionality.

The undesired behavior can be tested with this repo (instructions there).

I propose to add the functionality into the watch function, or at least propose a mechanism so that tooling authors can reuse the functionality.