TodoList - Mark completed task incomplete

When the user taps/clicks completed task, it will now be marked as
incomplete. (Removes strike through & changes the priority color,
setting to a default of low priority).

- Updating TableVC didSelectRowAtIndexPath method so we check to see if
the task is completed, and then set it as incomplete with a default
priority (and we slide in the change priority buttons so the user can
change from default priority).