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).

private/email-templates/verify-email.txt

@@ -0,0 +1,8 @@
+Hey, {{firstName}}!
+
+Can you do us a favor and verify your email address?
+
+[Verify Email Address]({{verifyUrl}})
+
+Thanks!
+{{applicationName}} Team