feat: exposing vips_gaussnoise operation

[alza54]

Gaussian noise feature respecting the proposed API

sharp({
  create: {
    width: 1024,
    height: 768,
    channels: 1,
    noise: {
      type: 'gaussian',
      mean: 128,
      sigma: 30
    }
  }
});

[coveralls]

Coverage remained the same at 100.0% when pulling 0f03310 on alza54:master into 138e60a on lovell:master.

[lovell]

Fantastic, thank you very much for this. I've left a couple of questions/comments inline that will probably need addressing, then we can merge this useful feature.

[lovell]

Marvellous, thank you very much.