This patch allows indenting task line as subtask using 4 initial
spaces. First all the tasks are parsed and indention stored,
after which the subtask heirarchy is resolved using indent
level.

Some heuristics:
    1) The difference between current line indent and previous
    line indent cannot exceed 1, if it does, the difference is
    taken 1.

    2) Tabs should not be used for indenting subtask. If tabs are
    present the parsing is stopped and a warning generated.