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.

cypress/integration/module_spec.js

@@ -0,0 +1,9 @@
+describe('es6 module', function() {
+    it('loads', function() {
+        cy.visit('http://localhost:8080/cypress/test-pages/es6-module.html');
+        cy.get('#result').should(($el) => {
+            const count = $el.text().split(',').length
+            expect(count).to.equal(3);
+        });
+    })
+});