support $PLANFILE env variable

[motatoes]

Atlantis allows completely overriding plan or apply commands in a custom way: https://www.runatlantis.io/docs/custom-workflows.html#custom-init-plan-apply-commands

digger should support similar syntax where a workflow that specifies a custom command and outputs in to $PLANFILE should also appear in the comments. For example in this custom workflow:

workflows:
  myworkflow:
    plan:
      - run:  terraform init -input=false
      - run: terraform plan -input=false -refresh -out $PLANFILE

Current behaviour: if plan or apply is overriden it runs the command successfully but does not comment back the pan to the PR (since it did not read the plan successfully.

Expected bahvaiour: plan should succeed AND the reporter should send the results back to the comments