Add className option for toolbar items (#1951)

Allows for passing in a className when registering a toolbar button to allow to target via CSS.

An example:
```js
   Prism.plugins.toolbar.registerButton( 'my-button', {
        text: 'My Button',
        className: 'my-button',
    });
```
This will add the span element `<span class="my-button">My Button</span>` .

plugins/toolbar/prism-toolbar.js

@@ -14,6 +14,7 @@
 	 * @property {string} text The text displayed.
 	 * @property {string} [url] The URL of the link which will be created.
 	 * @property {Function} [onClick] The event listener for the `click` event of the created button.
+	 * @property {string} [className] The class attribute to include with element.
 	 */
 
 	/**
@@ -44,6 +45,10 @@
 					element = document.createElement('span');
 				}
 
+				if (opts.className) {
+					element.classList.add(opts.className);
+				}
+
 				element.textContent = opts.text;
 
 				return element;