Wildcard custom item fields

[KevinFerm]

Hey! I would like to be able to do something like a regex to match custom fields. Like the most obvious one would be

  const parser = new Parser({
    customFields: {
      item: '*',
    },
  });

I guess.

This is because I'm building a system which allows users to map custom properties on the fly through an interface, and trying to reparse the feed every time something changes seems unnecessary.

[rbren]

🤔 yeah I could see the usefulness here. Feel free to open a PR!