Support multiple runs of the action in the same job

Description

Somewhat naive attempt at allowing the action to run multiple times in the same job. By deleting the plan and log before running octodns-sync, we make the plan file available to later steps. I also added the plan output to the action for more flexibility. The ability to use multiple octodns_refs is not supported. The ref from the first run will be used for any subsequent runs.

Hope this adequately addresses the concerns in the issue. I'm very open to any change suggestions!

Motivation and Context

See #57

How Has This Been Tested?

Created a test workflow which runs the action twice in the same job, and then prints the plan output. The action run was successful

Screenshots (if appropriate):

Types of changes

[X] Bug fix (non-breaking change which fixes an issue)
[X] New feature (non-breaking change which adds functionality)
~Breaking change (fix or feature that would cause existing functionality to change)~

Checklist:

[X] I have read the CONTRIBUTING document.
[X] My change requires a change to the documentation.
[X] I have updated the documentation accordingly.
[N/A] All new and existing tests passed.

solvaholic / octodns-sync