Allow for referencing $HOME path cookbook dirs

*This is more of a RFC / proof of concept than a ready-to-merge PR.*

I'd like to reference my `~/.berkshelf/cookbooks` path in a Packer configuration file. This seemed impossible, so I cracked open [play.golang.org](http://play.golang.org) and some StackOverflow questions and came up with the below solution.

Ideally, this would be extracted into a method and apply to all variables where paths are accepted as input.