Feature request: allow user to specify directory for caching downloaded files

[saintaardvark]

Hi there -- I tripped over a problem while using the ChefDK on OS X. I was running a ChefSpec test for Debian 8.2. The platform file isn't included in the version of fauxhai included in the ChefDK, so fauxhai tried to download it...only it was unable to save it locally because the permissions were restricted:

Failures:
  1) neteng_network_tools::default On debian 8.0 installs hping3
     Failure/Error: runner.converge(described_recipe)
     Errno::EACCES:
       Permission denied @ rb_sysopen - /opt/chefdk/embedded/lib/ruby/gems/2.1.0/gems/fauxhai-2.3.0/lib/fauxhai/platforms/debian/8.0.json
     # ./spec/unit/recipes/default_spec.rb:26:in `block (6 levels) in <top (required)>'
     # ./spec/unit/recipes/default_spec.rb:30:in `block (6 levels) in <top (required)>'

It would be nice if fauxhai could save that somewhere else (~/.fauxhai/platforms, maybe?) -- either if configured, or just as a fallback mechanism if saving to the original directory failed.

I realize that this could also be directed at the ChefDK project to change permissions for the installation; however, I think this could potentially be useful for other fauxhai users too.

Thanks, and please let me know if you need any further information.