Make step outputs available

Currently, step outputs aren't accessible from outside of runiac/terraform. It would be nice if runiac could provide a mechanism so that after running steps the outputs from those steps were available in some way. Specific use case example:

We have runiac/terraform configured to provision a resource group and an app service in that resource group. The resources follow a specific naming convention, something like "app-dev-adam-abc123" for the app service. After creating the resources with runiac, we want to deploy a package to the web app, using azure CLI, i.e. az webapp deployment. The inputs to that command take a resource group name and a web app name. Rather than define the naming convention in two places, it'd be nice if we could just use the output from one of the runiac steps which would contain the web app name. (Additionally, we are actually suffixing resources names with a unique value generated in terraform, so we can't define the convention outside of relying on the output from runiac/terraform.)

Start at the runiac/terraform step boundary described in the issue and trace how step results are currently handled. Define a mechanism for a later step to consume Terraform outputs such as the web app name, then verify it supports the Azure CLI deployment example.