Capstanfile 'files' section does not recurse directories

[jayaras]

Being able to pick and choose directories (and their content) would be helpful. This could allow a 'build' entry to be something like 'npm build' and not require a helper Makefile/script to build and then copy all the contents into ROOTFS.

Looking at the code its a trivial fix but not sure of the best way to make it happen. Always recurse directories? add another 'directories' section to the Capstan file which it recurses? With a little direction would be more than happy to take a stab at this and submit a PR.