Warn users when a PR contains some characters
    
    Unicode bi-directional characters can be present but unseen and thus
    missed during the review.
    With this PR, we create a list of characters that we want to warn the
    users about if present in a PR. Since that list is configurable, it
    can be extended as needed/desired.
    
    This relates to the CVE: CVE-2021-42574 and CVE-2021-42694
    More can be found about it at:
    https://access.redhat.com/security/vulnerabilities/RHSB-2021-007
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>