`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.
