Add -x option for custom output

[Jython1415]

This PR adds --execute (or -x) to run a specified command on the file and use the output instead of just reading the file.

My initial motivation was because I wanted to have a token limit (like with ttok -t) to be applied to each file, but after trying to bake that behavior into files-to-prompt I realized it would be simpler and more extensible to have files-to-prompt provide the --execute as the interface for further customization.

Here are some examples of how it could be used. I'm sure there's a bunch I'm not creative enough to think of at the moment.

# token-based limit
files-to-prompt . --execute "ttok -t 100 -i"
# line-based limit
files-to-prompt . --execute "head -n 10"

This doesn't change the default behavior, and I added new tests for the behavior with --execute.

Would love to hear feedback. If this isn't in-scope that's alright too. I just think it would be a good way to enable a lot of behavior that would otherwise be out-of-scope (e.g. #37 could be addressed with a script that does special behavior on .ipynb files).