Add a defaults parameter to templates.

[cmungall]

This allows default values to be filled for templates.

For example:

prompt: Summarize this text in the voice of $voice
defaults:
  voice: GlaDOS

When running without -p it will choose the default:

curl -s 'https://til.simonwillison.net/macos/imovie-slides-and-audio' | \
  strip-tags -m | llm -t summarize

But you can override the defaults with -p:

curl -s 'https://til.simonwillison.net/macos/imovie-slides-and-audio' | \
  strip-tags -m | llm -t summarize -p voice Yoda

I got this:

Text, summarize in Yoda's voice, I will: "Hmm, young padawan. Summary of this text, you seek. Hmmm. ...

[simonw]

This is a really good idea. I love that your PR includes the tests and documentation.