Add Dynamic Template Support

[thinkingserious]

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 USE_CASES.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

[devchas]

I can take a crack at this one!

[birjup]

Hi,
Will you do it something like this, or are you planning anything drastically different?

DynamicTemplateData map[string]string `json:"dynamic_template_data,omitempty"`

[birjup]

By the way, there seems to be a bug/feature where specifying html or text content stops the application of any handlebars

[devchas]

@birjup I am planning to do something like that

[devchas]

@birjup I actually had to do something like the following. You can't use a string value in the map because the JSON can be of arbitrary structure so that you can use the iteration and deep object features of handlebars. I am writing this so you pass a JSON string into the helper method

DynamicTemplateData map[string]interface{} json:"dynamic_template_data,omitempty"``

[birjup]

Great, makes sense. Thanks for the update