Add Dynamic Template Support #591

Issue Summary

On 7/24/2018, our team publicly launched dynamic content for transactional templates. It is now available for all customers sending over v3 of our Mail Send API. Iterate over lists, handle conditionals and more, thanks to native support for a subset of Handlebars syntax!

More information can be found in our blog post announcement.

You can currently use this feature by manually creating the request body as shown here.

Now, we need to create helper code and examples for this SDK.

Acceptance Criteria

• Implement a helper similar to what we have for the legacy templates
• Update the transactional_templates.md example to demonstrate the new Dynamic Templates using the helper and re-name the current example to Legacy

Documentation

• Reference Implementation
• How to Send an Email With Dynamic Transactional Templates
• Using Handlebars
• Interactive API Docs