Add a way for contributing file in config to be specified with local or global path

[RichardLitt]

I'm not sure how to do this in JavaScript. I want these two different solutions to work:

$ ls
contributing.md
config.json # Refers to contributing.md as `contributing.md`
$ build-a-space -c config.json
# Uses contributing.md

$ ls 
config.json # Refers to contributing.md elsewhere using global path
$ build-a-space -c config.json
# Knows contributing is global, but is still able to refer to it.

Right now, I can only put config.json in the build-a-space dir, and contributing.md's path has to be relative to basedir. This is not ideal.