Add a simpler exercise for demo

[kevinsimper]

Right now the only exercise is a demo where the output stream is replace by a through2 stream, but it is not really explained why.

Could there be a simpler demo that would lead up to this?

[linclark]

I'm not active in nodeschool anymore. I'm going to leave the repo as it is to match the post, but if someone wants to fork this and create a new and improved set of documentation, that's fine by me.