use a state monad for keeping track of the output state

I talked a little about this in my response in #24.

This would look similar to how it is done in [hindent](https://github.com/commercialhaskell/hindent/blob/1e8b036c41fdf517fdb40f06bd1d60a6a95c4393/src/HIndent/Types.hs#L33-L57).