Don't add the comment if it already exists

[nesl247]

Rather than requiring regex to delete comments, it would be great if this would automatically check if the comment exists, and if so, do not add a further comment.

For example, when setting up a PR review environment, we don't want it to constantly post the URL to the PR. We only need it done once.

[MPV]

I suppose you could use either -delete-comment-regex?

...or the newly added -edit-comment-regex, from:

• https://github.com/cloudposse/github-commenter/pull/26