Custom naming function setting

[Zirafnik]

I propose another setting to be added for the option of providing a custom naming function. This function would allow you to create custom image names, which completely override the outputFileMask (see #51) and the numbering system (see #52).

// Settings object
{
   namingFunction: (num) => {
       // do something and return a custom string
       return `squared_numbers_${Math.sqrt(num)}`;
   }
}

// Output for 2nd image: squared_numbers_4.png