make tasks.txt support more than two levels

[Mykola Kharechko]

I am trying to integrate gtimelog with basecamp within remote_tasks.txt. The problem i has - basecamp (http://www.basecamphq.com) projects has three levels (project->todolist->todoitem) but gtimelog supports only two levels (project->todoitem). It will be good if tasks.txt support yaml format and has arbitrary number of levels. Something like this:

- project 1:
    - todo list 1:
        - todolist item 1
        - todolist item 2
    - todo list 2:
        - todolist item 1
        - todolist item 2
            - sub todo list item 1
        - todo list without todoitems

- project 2:
    - todo list 1:
        - todolist item 1
        - todolist item 2
    - todo list 2:
        - todolist item 1
        - todolist item 2

[Mykola Kharechko (crchemist-quintagroup)]

Attached patch allows tasks.txt has more then two levels in this format:
level10:level2:level30:level40
level10:level2:level31
level11:level21:level30
...

With this patch all tests passed

[Sam Roberts (vieuxtech)]

I don't use the todo feature of gtimelog anymore, didn't find it useful (most of my todo is in github).

That said, it looks like the http://todotxt.com/ todo file format would be the best fit for gtimelog, both are agressively text based and editor friendly, and the todo.txt format has lots of interoperative tools.