Add Export Service

Provides library code to generate a Chef Zero compatible chef repo from
a Policyfile.lock.json. The intended use case for this is to copy the
code so Test Kitchen can test it, but it can also be used for
chef-solo-esque workflows with policyfiles.
Add export CLI
Add force option to policyfile export

Export now won't run if export dir isn't empty, unless force is given.
Add --force option to export command
Add tests for export CLI
Remove TODO for de-duplicating data item code.

Correctly de-duplicating is a bit involved and the code is related to
"compatibility mode," which will be removed once server support is
added. So we can live with the duplication for now.
Set CLI option to boolean