Feature: Add support for dynamic template variables

[davidyell]

The sendgrid/sendgrid library supports the interpolation of templates using dynamic tags and uses the $email->addDynamicTemplateData("subject1", "Example Subject 1"); method to add data to the email.

It would be nice if the Cake Email viewVars property could be adapted to this purpose.

Example https://github.com/sendgrid/sendgrid-php/blob/master/USE_CASES.md#kitchen-sink