Allow command arguments, file paths, and prompt messages to be templated

[LucasPickering]

More values in chains should be templated, specifically:

• The items in the command field of a !command
• The path field of a !file
• The message field of a !prompt

This will make chains much more composable. The work involved is:

• Change the model types from strings (or PathBuf) to Template
• Update render code in render.rs
  • The compiler errors caused by the model changes should point you in the right direction
• Write tests in template.rs
• Add documentation, via an example in templates.md