Document tape file syntax

[matt-gardner]

The tutorials are handy and give you plenty of examples of how to use various features in ducttape. But a complete syntax reference would be a very helpful addition to the documentation. It would let you see things like how to put all of the features together (the tutorial tape files tend to split them out), and let you see all of the options in one place, where you can then go look up more detail on a particular feature that you're not aware of. It would also let you know some important implementation specifics, like when newlines are allowed and when they aren't.