add_dynamic_template_data/3 has improper specs for nested template data

Typespecs are too strict for nested template data:
```
  @spec add_dynamic_template_data(t, String.t(), String.t() || map()) :: t
```

My usage:
```
|> Email.add_dynamic_template_data("item_title", item.title)
|> Email.add_dynamic_template_data("list_of_things", [
  %{"id" => item}
])
 ```
 This should probably be more permissive?