Add list files option

[kutsan]

We could do that with some grep tricks but it would be cool if we have --only-directories and --only-files options.

[thibran]

A -D flag for --only-directories would be nice.
Using the current exa -d */ feels a bit to cryptic for such a basic feature.

[sudo-nice]

~~To have exa --tree --only-directories, to grasp the overall directory structure quickly, would be very nice indeed.~~ Nevermind, it can be archived by tree -d.

[Motif23]

Adding a flag to view only files (not hidden) would be nice.

[zhengpd]

Nice to have those options although it can be done with tree -L 1 -d.