Add option keep_fargs.

By default it's `false`.  Pass `true` if you need to keep unused function
arguments.

Close #188.

lib/compress.js

@@ -61,6 +61,7 @@ function Compressor(options, false_by_default) {
         loops         : !false_by_default,
         unused        : !false_by_default,
         hoist_funs    : !false_by_default,
+        keep_fargs    : false,
         hoist_vars    : false,
         if_return     : !false_by_default,
         join_vars     : !false_by_default,
@@ -1044,6 +1045,7 @@ merge(Compressor.prototype, {
             var tt = new TreeTransformer(
                 function before(node, descend, in_list) {
                     if (node instanceof AST_Lambda && !(node instanceof AST_Accessor)) {
+                        if (!compressor.option("keep_fargs")) {
                             for (var a = node.argnames, i = a.length; --i >= 0;) {
                                 var sym = a[i];
                                 if (sym.unreferenced()) {
@@ -1058,6 +1060,7 @@ merge(Compressor.prototype, {
                                 else break;
                             }
                         }
+                    }
                     if (node instanceof AST_Defun && node !== self) {
                         if (!member(node.name.definition(), in_use)) {
                             compressor.warn("Dropping unused function {name} [{file}:{line},{col}]", {