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).

package.json

@@ -12,8 +12,11 @@
     "bcrypt": "^1.0.3",
     "commonmark": "^0.28.1",
     "core-js": "^2.5.1",
+    "fs": "0.0.1-security",
+    "handlebars": "^4.0.10",
     "jquery": "^2.2.4",
     "jquery-validation": "^1.17.0",
+    "juice": "^4.1.1",
     "lodash": "^4.17.4",
     "meteor-node-stubs": "^0.2.11",
     "prop-types": "^15.5.10",