Can't run call gulp-useref from parallel gulp workflows

I tried to run useref from two task in parallel, using the new gulp 4 _gulp.parallel_ method.
The result is that one of the tasks almost always gets an error (no stack),
or that I get the error **Error: stream.push() after EOF**.

After looking at index.js, I have a suspicion at to what the problem is:

``` javascript
streamManager = require('./lib/streamManager');

module.exports = function (options) {
streamManager.transforms = Array.prototype.slice.call(arguments, 1);
streamManager.options = options || {};

streamManager.additionalStreams();

return through.obj(streamManager.transformFunction, streamManager.flushFunction);
};
```

Instead of setting properties on a global streamManager object, you could just pass these in as arguments to a function?
If you approve, I'll try to find the time to submit a pull request this week.