Add CSVUtil command with uniform interface

Add a `CSVUtil::Comand` with a `#process` method that is implented by each of the following:

- [ ] `CSVUtil::Cat`
- [ ] `CSVUtil::Cut`
- [ ] `CSVUtil::Filter`
- [ ] `CSVUtil::Slice`
- [ ] `CSVUtil::Split`

The `#process` method should take no arguments and print its outpout to the provided output (STDOUT, StringIO, File, etc.).
