Set type="button" attribute for copy to clipboard plugin. (#2593)

When using Prism + copy to clipboard inside a HTML form, the form is submitted when copying.
It's because by default the button is seen as a submit button.
Adding the `type="button"` attribute prevents this.

Author: alexsegura

plugins/copy-to-clipboard/prism-copy-to-clipboard.js

@@ -38,6 +38,7 @@
 	Prism.plugins.toolbar.registerButton('copy-to-clipboard', function (env) {
 		var linkCopy = document.createElement('button');
 		linkCopy.textContent = 'Copy';
+		linkCopy.setAttribute('type', 'button');
 
 		var element = env.element;