No way to merge with sass stream

I would like to use `gulp-wpstylecss` with `gulp-sass`:

```
return wpstylecss()
.pipe(addsrc('style.scss'))
.pipe(sass())
.pipe(gulp.dest('./dest'))
```

This code should output style.css with header generated by `gulp-wpstylecss` and css as result of sass compilation. Unfortunately it results with css only. No header at all.
