Add postconf option support

[hkcomori]

Set Postfix settings like smtp_user.

$ sudo docker run -p 25:25 \
    -e maildomain=mail.example.com -e smtp_user=user:pwd \
    -e postconf=key:value \
    --name postfix -d hkcomori/postfix
# Set multiple configs: -e postconf=key1:value1,key2:value2,...,keyN:valueN
# Restrictions: key, value must not contain `,`, `:`