add envelope to email node options

as part of node-red#875

social/email/61-email.js

@@ -89,6 +89,7 @@ module.exports = function(RED) {
                     var sendopts = { from: node.userid };   // sender address
                     sendopts.to = node.name || msg.to; // comma separated list of addressees
                     sendopts.subject = msg.topic || msg.title || "Message from Node-RED"; // subject line
+                    if (msg.hasOwnProperty("envelope")) { sendopts.envelope = msg.envelope; }
                     if (Buffer.isBuffer(msg.payload)) { // if it's a buffer in the payload then auto create an attachment instead
                         if (!msg.filename) {
                             var fe = "bin";