`ls --classify` prints extra characters that are helpful to users.
Users may wish to turn that flag on by default (e.g. through a shell
alias or `LS_OPTIONS`). However, it is unfriendly to use it in
combination with other commands (e.g. `alias ls='ls --classify'; ls |
grep '.sh$'`).
Add an optional argument to `--classify` indicating _when_ to classify,
just like the optional argument for `--color`. This lets users use `ls
--classify=auto` to prevent the classify indicator style from activating
when the output is not to a TTY.
Add a test `tests/ls/classify.sh`, and update documentation.