Support for images through standard input about smartcrop.js

[jwagner]

smartcrop-cli already does this:
https://github.com/jwagner/smartcrop-cli/blob/master/smartcrop-cli.js#L139

jonas@fractal:~/Pictures/2017/04/13$ smartcrop - < 1290r_small.jpg 
{
  "topCrop": {
    "x": 504,
    "y": 0,
    "width": 1536,
    "height": 1536,
    "score": {
      "detail": 3275.496181869063,
      "saturation": 677.1039681853479,
      "skin": 442.4354769055732,
      "boost": 0,
      "total": 0.0007013169544048092
    }
  }
}