Add option to parse newer EmailMessage type from mails

Add option to parse newer EmailMessage type from mails

Sofar, parser policy was not specified, so "compat32" was used for compatibility
with python 3.2. Changing to the more modern "default" policy returns a different
type of message (EmailMessage instead of Message) that has quite some differences.
For example, EmailMessage.get(header_name) returns no longer str but a Header object.
This would break lots of code so use the old compat32 as default still.