Create and map bare-returns into new parse property name

* Following as much previous structure as possible... see analysis at https://github.com/mishoo/UglifyJS2/issues/935#issuecomment-180712909
* Using `undefined` as default since is technically assigned elsewhere
* Note entirely sure if https://github.com/mishoo/UglifyJS2/blob/master/test/compress/return_undefined.js is the test for this or not
* Change README.md to reflect and add additional sub-note about Userscript IIFE possibility

Applies to mishoo/UglifyJS2#935

README.md

@@ -125,7 +125,9 @@ The available options are:
   --noerr                       Don't throw an error for unknown options in -c,
                                 -b or -m.
   --bare-returns                Allow return outside of functions.  Useful when
-                                minifying CommonJS modules.
+                                minifying CommonJS modules and Userscripts that
+                                may be anonymous function wrapped (IIFE) by the
+                                .user.js engine `caller`.
   --keep-fnames                 Do not mangle/drop function names.  Useful for
                                 code relying on Function.prototype.name.
   --reserved-file               File containing reserved names
@@ -654,6 +656,8 @@ properties are available:
 - `filename` — the name of the file where this code is coming from
 - `toplevel` — a `toplevel` node (as returned by a previous invocation of
   `parse`)
+- `bare_returns` — Allow return outside of functions. (maps to the
+  `--bare-returns` CLI arguments option)
 
 The last two options are useful when you'd like to minify multiple files and
 get a single file as the output and a proper source map.  Our CLI tool does

tools/node.js

@@ -40,7 +40,8 @@ exports.minify = function(files, options) {
         warnings     : false,
         mangle       : {},
         output       : null,
-        compress     : {}
+        compress     : {},
+        parse        : {}
     });
     UglifyJS.base54.reset();
 
@@ -60,7 +61,8 @@ exports.minify = function(files, options) {
             sourcesContent[file] = code;
             toplevel = UglifyJS.parse(code, {
                 filename: options.fromString ? i : file,
-                toplevel: toplevel
+                toplevel: toplevel,
+                bare_returns: options.parse ? options.parse.bare_returns : undefined
             });
         });
     }