Do not count line endings toward character limit

[mvz]

With the existing implementation, poper would incorrectly flag lines that have exactly 72 character but are followed by a newline.

This pull request

• Adds specs to test the case of exactly 72 characters;
• Adds specs so the case where the long line is followed by a newline is tested.
• Amends the CharacterLimit rule to remove line endings from the message lines.

[mmozuras]

@mvz perfect, thanks 🙇