Support HTML5 comment syntax (enabled by default!)

See http://javascript.spec.whatwg.org/#comment-syntax
    https://github.com/mishoo/UglifyJS/issues/503
    https://github.com/marijnh/acorn/issues/62

lib/output.js

@@ -399,7 +399,7 @@ function OutputStream(options) {
                     });
                 }
                 comments.forEach(function(c){
-                    if (c.type == "comment1") {
+                    if (/comment[134]/.test(c.type)) {
                         output.print("//" + c.value + "\n");
                         output.indent();
                     }