async function main(oldPath, newPath) {
  const oldScreenshot = await Jimp.read(oldPath);
  const newScreenshot = await Jimp.read(newPath);
  const ssimResult = ssim(oldScreenshot.bitmap, newScreenshot.bitmap);
  console.log(ssimResult.mssim);  // A score between 0 and 1.
}
Compare two images and output the similarity between 0 and 1.  Uses the same
comparisons done in the tests through Karma.

@param {string} oldPath
@param {string} newPath