implement --help options (#2017)

tools/node.js

@@ -63,3 +63,20 @@ function describe_ast() {
     doitem(AST_Node);
     return out + "\n";
 }
+
+function infer_options(options) {
+    var result = UglifyJS.minify("", options);
+    return result.error && result.error.defs;
+}
+
+UglifyJS.default_options = function() {
+    var defs = {};
+    Object.keys(infer_options({ 0: 0 })).forEach(function(component) {
+        var options = {};
+        options[component] = { 0: 0 };
+        if (options = infer_options(options)) {
+            defs[component] = options;
+        }
+    });
+    return defs;
+};