Add support for deleting workflow run logs

[nightlark]

Blog post: https://github.blog/changelog/2020-04-21-github-actions-logs-can-now-be-deleted/
API docs: https://developer.github.com/v3/actions/workflow_runs/#delete-workflow-run-logs

I'll open a PR for this soon.

[gmlewis]

Thank you, @nightlark !