add indentation character option

Ablt to render a tree with character like `|` or `│`, e.g.

```bash
$ treeless -i 4 -C '│' -p -D '\.git' -F '.*\.pyc'
.gitignore
LICENSE
MANIFEST.in
README.rst
setup.py
scripts/
│   treeless
treeless/
│   __init__.py
│   lesskey.bin
│   lesskey.txt
│   treeless.py
```