renamed --screw-ie to --screw-ie8, documented it in README.md, indicated it doesn't break IE9+

lib/scope.js

@@ -64,7 +64,7 @@ SymbolDef.prototype = {
     mangle: function(options) {
         if (!this.mangled_name && !this.unmangleable(options)) {
             var s = this.scope;
-            if (this.orig[0] instanceof AST_SymbolLambda && !options.screw_ie)
+            if (this.orig[0] instanceof AST_SymbolLambda && !options.screw_ie8)
                 s = s.parent_scope;
             this.mangled_name = s.next_mangled(options);
         }
@@ -343,7 +343,7 @@ AST_Toplevel.DEFMETHOD("_default_mangler_options", function(options){
         eval     : false,
         sort     : false,
         toplevel : false,
-        screw_ie : false
+        screw_ie8 : false
     });
 });