dkim_sign: added config/dkim/+domain support
dkim: completed per-domain dkim key use
update DKIM doc.md, add comments in 'dns' file
dkim: check for higher level DKIM keys when signing

for example, when a message is being sent from mail.example.com, also
check for an example.com signing key.
dkim docs: formatting fixes
dkim docs: clarify single use config