Feature Request: nesting templates

It would be great if we could nest templates using a `nested_template` parameter configured in the YAML block.

What I thought of is something similar to dropdown, but for selecting templates instead of elements from a pre-defined list.
Afterwards, the plugin would read that file in and dump the contents when adding `{{nested_template}}`.
Of course, if the user wants add multiple nested templates he can just define extra yaml parameters for that.

Since we already have the option of inserting templates with a dropdown, I think we could also expose that functionality via a function that can be called in the YAML. I do not have the knowledge on the codebase (yet), but am eager to explore this if it is of interest. Let's talk about it.