Allow a transform function to be applied to each chunk

Sometimes we wish to load the data chunk by chunk and apply a function to it instead of loading it into memory. 

E.g. I may want to load each chunk and then save each function as a feather file. I would do it like this

```julia
readcsv("file/path.csv") do (chunk_id, chunk)
    Feather.save(chunk_id*".feather", chunk)
end
```