feat: Add module support with new dist files

- `/dist/color-thief.umd.js`: UMD module. For simple script tag loading that exposes a global variable or for RequireJS AMD support.
- `/dist/color-thief.js`: CommonJS module. Entry point for Node.js and Browserify.
- `/dist/color-thief.mjs`: ES6 module. For modern browsers as well as Webpack and Rollup.
- `/dist/color-thief.min.js`: Duplicate of `/dist/color-thief.umd.js`. Kept around to maintain backwards compatibility.

src/color-thief.js

@@ -1,5 +1,5 @@
 /*
- * Color Thief v2.1.0
+ * Color Thief v2.2.0
  * by Lokesh Dhakar - http://www.lokeshdhakar.com
  *
  * Thanks
@@ -641,3 +641,5 @@ var MMCQ = (function() {
         quantize: quantize
     };
 })();
+
+export default ColorThief;