Add --comment param to "jip send" to add a GH comment e.g. to explain why a PR was updated

[omarkohl]

If the PR is completely new then it should just be posted as a new comment. If the PR is being updated the comment should be incorporated into the auto-generated comment with the diff.

The main use case for this is when sending a single PR, but if specified the comment should be posted to all affected PRs. It's worth considering what should happen in a stack where only one PR actually gets updated even though multiple exist. Probably in that case the comment should only be posted to the PR being updated (updating the PR description with a different stack would not count as an update, only modifications that warrant a new push or a code diff).