New API to allow setting custom value display

[nodegin]

Hello, thank you so much for this awesome library!

It would be helpful to allow passing a function like:

valueFormatter: (value) => {
  if (typeof value === 'string') {
    if (value.length === 0) return ['yellow', '(empty string)']
  }
  if (typeof value === 'undefined') {
    return ['yellow', '(undefined)']
  }
  if (typeof value === 'number') {
    return ['white', value]
  }
  return ['green', value]
}