Issue#16

Add support for unordered list (at most 3 levels).
Effect:
```
TODO list:
* TODO major 1
    - TODO minor 1.1
    - TODO minor 1.2
* TODO major 2
* TODO major 3
    * TODO minor 3.1
    * TODO minor 3.2
```
becomes:
```
TODO list:
+-- TODO major 1
|   +-- TODO minor 1.1
|   `-- TODO minor 1.2
+-- TODO major 2
`-- TODO major 3
    +-- TODO minor 3.1
    `-- TODO minor 3.2

```