Add an option to omit default props that are empty function callbacks

[zeckdude]

Often times a callback prop is assigned an empty function (() => {}) as the default value. The showDefaultProps method is simply looking for equality (I believe), so it won't catch a default prop that is just an empty function. In those cases, I had to add some logic in the filterProps property to make those not appear but I don't think this is the best solution and I would love it if the showDefaultProps would catch those as well or if there was another option I can set so those empty functions don't show up in the generated string.

filterProps: value => {
  // Omit empty functions (they're likely from default props and `showDefaultProps` doesn't catch those)
  if (!isFunction(value)) {
    return true;
  }

  return value.toString() !== '() => {}';
},