add support for transactional html/text emails

- Add support for welcome email on user creation (both OAuth and password users)
- Add support for html/text templating on verify email.
- Add support for html/text templating on reset password.
- Add handlers for converting Handlebars templates to HTML and text (template agnostic).
- Add helper function sendEmail() for compiling and sending templates via Email.send() (SMTP).

imports/modules/server/send-email.js

@@ -0,0 +1,26 @@
+import { Email } from 'meteor/email';
+import getPrivateFile from './get-private-file';
+import templateToText from './handlebars-email-to-text';
+import templateToHTML from './handlebars-email-to-html';
+
+const sendEmail = (options, { resolve, reject }) => {
+  try {
+    Meteor.defer(() => Email.send(options));
+    if (callback) resolve();
+  } catch (exception) {
+    reject(exception);
+  }
+};
+
+export default ({ text, html, template, templateVars, ...rest }, callback) => {
+  if (text || html || template) {
+    return new Promise((resolve, reject) => {
+      sendEmail({
+        ...rest,
+        text: template ? templateToText(getPrivateFile(`email-templates/${template}.txt`), (templateVars || {})) : text,
+        html: template ? templateToHTML(getPrivateFile(`email-templates/${template}.html`), (templateVars || {})) : html,
+      }, { resolve, reject });
+    });
+  }
+  throw new Error('Please pass an HTML string, text, or template name to compile for your message\'s body.');
+};