Crate to write an ascii tree.

``` let l1 = Leaf(vec![String::from("line1"), String::from("line2"), String::from("line3"), String::from("line4")]); let l2 = Leaf(vec![String::from("only one line")]); let n1 = Node(String::from("node 1"), vec![l1.clone(), l2.clone()]); let n2 = Node(String::from("node 2"), vec![l2.clone(), l1.clone(), l2.clone()]); let n3 = Node(String::from("node 3"), vec![n1.clone(), l1.clone(), l2.clone()]); let n4 = Node(String::from("node 4"), vec![n1, n2, n3]);

let mut output = String::new(); let _ = write_tree(&mut output, &n4); ```

The result would be:

 node 4
 ├─ node 1
 │  ├─ line1
 │  │  line2
 │  │  line3
 │  │  line4
 │  └─ only one line
 ├─ node 2
 │  ├─ only one line
 │  ├─ line1
 │  │  line2
 │  │  line3
 │  │  line4
 │  └─ only one line
 └─ node 3
    ├─ node 1
    │  ├─ line1
    │  │  line2
    │  │  line3
    │  │  line4
    │  └─ only one line
    ├─ line1
    │  line2
    │  line3
    │  line4
    └─ only one line