use file.clone()

I added a `file.clone()`  to enable multiple stream to work on the same file.
this would let me do something like this
What do you think?

```js
  var svg = gulp
    .src('images/svg/*.svg');

  var png32 = svg
    .pipe(rsvg({ width: 32, height: 32 }))
    .pipe(gulp.dest('images/png-32'));

  var png64 = svg
     .pipe(rsvg({ width: 64, height: 64 }))
     .pipe(gulp.dest('images/png-64'));

  return es.merge(png32, png64);
```