[feature] Write `octodns-sync` plan to pull request comment

[solvaholic]

fixes #35

How to test with the changes in this pull request

Use solvaholic/octodns-sync@issue35 to run the changes pushed to branch issue35:

• Add an html output to your plan_outputs in your octodns config, for example in public.yaml:

  manager:
  plan_outputs:
    html:
      class: octodns.provider.plan.PlanHtml

• Configure your workflow to enable add_pr_comment and provide a token:

  on:
  pull_request:
  jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - uses: solvaholic/octodns-sync@issue35
        with:
          config_path: public.yaml
          add_pr_comment: 'Yes'
          pr_comment_token: '${{ github.token }}'

To-do list

• [x] Document feature in README
• [x] Document changes in CHANGELOG
• [x] Save octodns-sync log and plan output to two separate files
• ~[ ] Expose those ☝️ file paths as action outputs~
• [x] Add action inputs to enable feature and to define an API token
• [x] Add octodns-sync plan as a comment to the triggering pull request when:
  • GITHUB_EVENT_TYPE is pull_request
  • Input add-pr-comment is "Yes"
• [x] What changes are required for run local option?
• [x] Provide example configs for different usage scenarios?

[solvaholic]

Expose those ☝️ file paths as action outputs

I changed my mind, would like to do this step in a separate pull request. It's unnecessary for this feature and it'll require changes to entrypoint.sh (and, therefore, the Docker image).

[solvaholic]

What changes are required for run local option?

None.

Provide example configs for different usage scenarios?

I added to README.md to cover available features.

[solvaholic]

I'll go ahead and merge this into main. Manual tests went fine and the new feature is disabled by default.

[xt0rted]

@solvaholic are you planning on doing a new release with these changes or are you waiting until the feedback I had is addressed? Right now I'm pinning to a4b4942c255994ea2c6f193dfc472dec8093b219 and did my first deploy Friday. It worked great, thanks again for the great action.