Add setting to load Inspec plugins for additional functionality

[thheinen]

Description

Currently, kitchen-inspec does not support InSpec plugins, as it directly calls the Runner. This PR implements a setting to enable loading of plugins (like additional inputs). The setting defaults to false to not break compatibility.

Issues Resolved

No filed issue so far.

Check List

• [X] All commits have been signed-off for the Developer Certificate of Origin. See https://github.com/chef/chef/blob/master/CONTRIBUTING.md#developer-certification-of-origin-dco
• [X] PR title is a worthy inclusion in the CHANGELOG