Partial matching of Kitchen instance names

By parsing `.kitchen.yml` for a list of all available instances and searching
the list for the provided instance name using partial matching.

Partial matching allows the user to specify only a portion of the instance
name. For example, `chef-runner -K ubuntu` will provision the instance
"default-ubuntu-1404" if that's the first instance with
"ubuntu" in its name.

Fixes #23.