Consolidate input and output handling

[demery]

Consolidate input and output handling.

• Have common input selection for scripts with similar IO profiles:
  • Accepts CSV from STDIN or as file argument
    • csv-slice
    • csv-filter
    • csv-cut
  • Accepts a list of CSVs as file arguments or a piped list of files
    • css-cat
      • Question: Instead of as a list of CSVs as it does now, should csv-cat treat input from STDIN as a CSV?
• Have all scripts use a common -o --output FILENAME method for selecting out; default should be STDOUT

[demery]

csv-cat: The unix cat utility reads input from standard in as a file. The correct approach may be to do the same here for consistency. However, csv-cat also does some things cat does not: sort and uniqueness operations. Should those behaviors be moved to separate programs?