I'm trying to use the `--mange-regex` option which is documented on UglifyJS2's GItHub README. The problem is, my build uses `gulp-uglify`, and it isn't clearly documented how I should use `--mangle-regex` without the command line.

My goal is to mangle 'private' identifiers (matching `/^_/`)

I've tried: 

```
// Gulp setup code left out for brevity (I'm using gulp.src() etc properly)
uglify({
    mangle: {
        toplevel: false
    },
    mangleProperties: {
        regex: /^_/
    }
});

```

and 

```
// Gulp setup code left out for brevity (I'm using gulp.src() etc properly)
uglify({
    mangle: {
        regex: /^_/
    }
});

```

Nothing's doin' here. A little help?