We can't assume line endings of specific platforms but should be
flexible with that when detecting the signature part of an email.

Also this fixes the styling of signatures a bit. They looked quite off
now that the distinct styling came back.

Fine-tuning in terms of design can be done in follow-ups. This PR is
mainly about getting the detection back.

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>