Add XOR formula

[Nicolas Cadier]

Title says it all, a XOR formula would be really helpful and reduce formula length when trying to test an exclusive or.

For those unfamiliar, here is the logic table for XOR. It acts as an OR however if both inputs are TRUE, then XOR returns false.

[JaredDolich]

@Nicolas Cadier 

One use case I can think of that might take advantage of this is when I need to make sure I have one, and only one, TRUE. I do this all the time for validation work. Would you envision this working across a list? Meaning it tests all the list items and only returns a TRUE if only one of the conditions is TRUE?