No more flags dependency. Flag handling closer to cut: `csv-cut -f1,3`
Use struct to hold program arguments.
Adds TODO note.
Move functionality out of main to be testable.
Testing cut functionality against single and multiple files.
Use assert and move actual one new line.
Default to Stdin for input when no files are given.
Filename - defaults to Stdin.
Factor out input opening.
Rename fields to ranges.
Towards ranges.