todo-txt: add subtask implementation
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.
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