Add ability to add to default callback rather than replacing it

[Daniel15]

If you don't pass a done function, webpack-stream provides its own: https://github.com/shama/webpack-stream/blob/master/index.js#L33-L66

I want to reuse webpack-stream's logic and add my own small bit of code. To do this at the moment, I'd have to copy and paste the block of code from webpack-stream, as it does not provide any way to run the default callback. Please make this easier :)