associate cache dir with the output format as well

I think the cache dir should be associated with a specific output format, otherwise the cache can be frequently invalidated when switching between different output formats because each output format has its own set of chunk options.

I put `output_format$pandoc$to` in `pandoc_to` since there are multiple instances of it. If you do not like it, I can remove it and make this PR a one-liner change.