Group results according the indicators appended by -F/--classify (ls --sort=class)

[aristidesfl]

> From the man entry:
```
-F, --classify
              append indicator (one of */=>@|) to entries
```

Would be useful for readability purposes to be able group entries 
according the indicators appended by -F/--classify.
A WORD called class could be added to "--sort" for this purpose.

In addition to that, it would be useful to have an option which forced the 
extra entries printed by -a/-A, to be displayed after the rest of the normal 
entries, independently of sorting options.


Cheers,
aristidesfl