Export Policyfile as Chef Zero-compatible repo

Export a policy as a Chef Zero compatible repo. The immediate goal for this work is to enable policyfiles to be run in TK via a TK provisioner (will be created after this is merged), but it also enables chef-solo-style workflows to be used by packaging and distributing policy repos via an out-of-band mechanism and running chef-client in local mode.

Looking for feedback on the `force` option: is it too strict? Should `--force` only be required if a `cookbooks` and/or `data_bags` directory is present? Should the cleanup that `--force` implies only affect those directories?

@opscode/client-engineers 
I added it to the `force` option, but I don't know if it actually does anything. When I added some debug code to print the values of the mixlib-cli `config` object, here's what I get:

```
$ bin/chef export -D
{:config=>{:force=>false, :debug=>true}}
$ bin/chef export   
{:config=>{:force=>false, :debug=>false}}
```