[Feature] Niffy should accept and test multiple resolutions

[mousemke]

What needs to get done?

• niffy should take an object as an option with different resolutions to test all. example:

const niffyTargets = {
  xl: [1205,600],
  l: [965, 600],
  m: [605, 600],
  s: [325, 600]
}

const niffy = new Niffy( url1, url2, { 
    targets: niffyTargets,
    pngPath: path.resolve(process.cwd(), `./niffy`)
 });

in tihs case, it whould output diff images to the following directories:

• ./niffy/xl
• ./niffy/l
• ./niffy/m
• ./niffy/s

Why does it need to get done?

it saves everyone making their own implementation of the loop and streamlines the process

How do I do it?

loop nightmare over the keys