Add new option to show the size of the files

A functionality of displaying the size of the files is in original `tree` command like this.
I'd like to get the same function

```console
$tree internal/ -h
[ 160]  internal/
├── [ 542]  counter.go
├── [ 505]  counter_test.go
└── [1.8K]  gcstree.go

1 directory, 3 files
```