Add support for a post_create_command

There is already a `pre_create_command` that executes a custom command prior to the `vagrant up` command. This pull request adds support for a `post_create_command` as well.

I believe supplying something like `vagrant ssh -c 'some command'` to `post_create_command` could resolve https://github.com/test-kitchen/kitchen-vagrant/issues/78.