replacing src with object

Sometimes you want to include other files into metadata, like cover
images or subtitles. This allows optional inclusion with the default
format header
Let's not modify the `opts` object from what the user gives us. We don't know what the use may expect to do with this object in the future, so we shouldn't write to it. Keep `dst` as a private variable and pass it to `spawnWrite` as a separate parameter.