Add rule to warn with unoptimized files

[ramasilveyra]

Name: images/unoptimized

Reference: https://github.com/sindresorhus/gulp-imagemin/

Also add a rule option to pass custom imagemin options, maybe something like pointing to a file with the custom options. E.g.:

    "images/unoptimized": [true, {
      "imagemin-options-file": "./config/imagemin.js"
    }]

./config/imagemin.js:

'use strict';

const { gifsicle, jpegtran, optipng, svgo } = require('stylelint-images');

module.exports = [
  gifsicle(),
  jpegtran(),
  optipng(),
  svgo({ plugins: [{ removeUselessDefs: false }] })
];

Also cache the optimized result.