Support multiple destinations in copy-files.js

**I'm submitting a feature request**

**Current behavior:**
The copy-files method only supports a single destination. Because the instruction list is normalized you cannot add duplicate destinations by copying the same file twice. My particular application requires some files to be copied to multiple locations for different builds. In order to achieve this I have to have multiple copy-file routines in my code base.

* **What is the expected behavior?**

It would be beneficial to have the copy-files method accept an array of destinations for each instruction instead of a single string. The method would copy each file to each location in the instruction array, thus eliminating the need to replicate the copy-file routine.

* **What is the motivation / use case for changing the behavior?**
Increased flexibility in the build process, lower the need for hacks for special requirements, and a cleaner code base.

Start by locating copy-files.js and the copy-files method, then inspect how its instruction list currently handles a single destination. Done means each instruction can accept an array of destinations and the source file is copied to every listed location without requiring duplicated copy-file routines.

Written by the indexing model from the issue text.