Feat: Implement Block-Comment

[superstes]

As this role already implements some ways of blocking traffic, it would make sense if we'd add a block-comment to the logs.

• Add the txn.block_comment variable to the log-format (+ document it for users with custom log-formats)
• Update the variable-content before a block would match
• Allow users to disable block-comments as it might make the generated config a little harder to read/understand