Move template file to yaml format

[dpolhamus]

Instead of a named list per the current setup, a yaml would be substantially easier for users to manipulate outside of the app.

Bonus would be we could offload creation of these files and handling of more complex objects to something like the r-yaml package

[dpolhamus]

We'll also provide a function to convert old templates into yaml format for backward compatibility.

[dpolhamus]

Est 34 hrs