Document passings args to formatters

[jeddy3]

Which issue, if any, is this issue related to?

Closes #4668

Is there anything in the PR that needs further explanation?

Keeps it simple. We can always expand on it later if there's a need.

[hudochenkov]

It's not clear for me how piping output is passing arguments to formatter. Piping happens after formatter was ran.

[jeddy3]

You're right. It's not clear that the second approach is a separate formatting program rather than a stylelint formatter. I've updated the docs to hopefully illustrate that.

---

Example formatting program:

// my-program-that-reads-JSON.js
const getStdin = require("get-stdin");
(async () => {
  const jsonString = await getStdin();
  const object = JSON.parse(jsonString);
  let colors = false;
  if (process.argv[2] && process.argv[2] === "-c") colors = true;
  console.dir(object, { depth: null, colors });
})();

stylelint -f json "*.css" | my-program-that-reads-JSON --c

This just (optionally) colourises and indents the JSON, but a formatting program could do anything with the data.