add reading password from file

Signed-off-by: Florian Brandes <florian.brandes@posteo.de>

config.example

@@ -6,7 +6,8 @@
 hostname = smtp.mail.example.com
 port = 465
 username = foo@example.com
-password = s3cr3t
+# If password_file is supplied, a valid path must be supplied
+# password_file = Path/to/file
 sender = foo@example.com
 # set_reply_to = false
 # use_tls = true