Allow defining a filter/preprocessor

[xeruf]

i.e. a command that processes the input before it is displayed by bat.

I often want bat's nice filename display, ruler and line number, but can't give it the file directly.
For example in #2024 I wanted to use cut, now I am building something with python -m json.tool to format json before displaying it in bat with syntax highlighting. By adding the ability to process the input, one could use both nicely :)

So rather than:

cat "$@" | python -m json.tool | bat --language json --style numbers

I could use something like:

bat --language json --style header,rule,numbers --process 'python -m json.tool' "$@"

This way I can distinguish the files.

I guess the processor should be invoked line-based, or there could be a separate --process-lines option.

[keith-hall]

the proof of concept of bat plugins would likely fit this use case: #2203

[xeruf]

I just encountered a good use-case again, where I would like to reproduce git log -p in fossil, viewing each commit like a separate file in bat ^^