Add an argument validator function to tasks and facts

Currently, tasks and facts can have minimum required arguments, so that the command line doesn't let users create nonsense investigations. There may be situations where the set of valid arguments is fairly restricted, and using a function for this would let fact/task authors define that set from the beginning. The cli could then call this function to ensure the input is valid, and the daemon could call this same function before evaluating a fact or running a task.