moved 'exit' function to main.
uncommented the markDone function. closes #11.
(debugged) changed task indexes to i-1 instead of i when processing.
(debugged) adding task: checked for desc == null instead of desc != null
fixed edit function (from commit c077ed23e06f29c7b04ca292a8ba378d816a75a3).
added (temporary) "magic" strings for feedback purposes.
closes #22