proposal: Comment header and footer

[peaceiris]

Is your feature request related to a problem? Please describe.

This was suggested by @Andre601 at #308 (comment)

Describe the solution you'd like

# .github/label-commenter-config.yml

comment:
  header: Hi, there.
  footer: >-
    > This is an automated comment created by the [actions-label-commenter](https://github.com/peaceiris/actions-label-commenter).
    Responding to the bot or mentioning it won't have any effect.

labels:
...

Describe alternatives you've considered

N/A

Additional context

N/A

[github-actions]

Thank you @peaceiris for suggesting this.

Log | Bot Usage

[peaceiris]

Closed by #327