No need to link to a directory

[murata100]

For the following directories and files.

# tree 
.
├── aaa
│   └── bbb.html
└── xxx
    └── yyy
        └── zzz.html

3 directories, 2 files

Now markdown-tree outputs:

## [aaa](aaa)
- [bbb.html](aaa/bbb.html)
## [xxx](xxx)
- [yyy](xxx/yyy)
  - [zzz.html](xxx/yyy/zzz.html)

No need to link to a directory.
In most cases, even if you browse the directory, the list of files will not be displayed (maybe 404 NotFound).

## aaa
- [bbb.html](aaa/bbb.html)
## xxx
- yyy
  - [zzz.html](xxx/yyy/zzz.html)

Is it possible to suppress with the option flag?