Mark a todo item as completed

[avinashbest]

• Use the done command to mark a todo item as completed by its number.
  $ ./todo done 1 Marked todo #1 as done.
• Attempting to mark a non-existed todo item as completed will display an error message.
  $ ./todo done 5 Error: todo #5 does not exist.
• Refer Line 124 todo.cpp