Add a CLI argument to allow sprite index file to be minified

[flother]

When a sprite index file is output by Spreet, it's pretty-printed. There should be an option to allow it to be output without any optional whitespace. For example, this should be the default output:

{
  "poi": {
    "width": 32,
    "height": 32,
    "x": 0,
    "y": 0,
    "pixelRatio": 1
  }
}

But when the --minify-index-file argument is passed it should be output as:

{"poi":{"width":32,"height":32,"x":0,"y":0,"pixelRatio":1}}