expand symbol space to improve compression (#2460)

- give globally distinct names to distinct variables
- improve ability to compress cross-scoped
- introduce `options.rename` to `minify()`
- default `true` if both `compress` & `mangle`

bin/uglifyjs

@@ -47,6 +47,7 @@ program.option("-d, --define <expr>[=value]", "Global definitions.", parse_js("d
 program.option("--ie8", "Support non-standard Internet Explorer 8.");
 program.option("--keep-fnames", "Do not mangle/drop function names. Useful for code relying on Function.prototype.name.");
 program.option("--name-cache <file>", "File to hold mangled name mappings.");
+program.option("--no-rename", "Disable symbol expansion.");
 program.option("--self", "Build UglifyJS as a library (implies --wrap UglifyJS)");
 program.option("--source-map [options]", "Enable source map/specify source map options.", parse_source_map());
 program.option("--timings", "Display operations run time on STDERR.")
@@ -65,6 +66,7 @@ if (!program.output && program.sourceMap && program.sourceMap.url != "inline") {
     "compress",
     "ie8",
     "mangle",
+    "rename",
     "sourceMap",
     "toplevel",
     "wrap"