Add an option to output the formula used to calculate the probability

[ajcr]

Here is a suggestion for a new feature from PilotPirx made in a comment here:

It would be awesome if there was also an output option in the calculator, that would provide the exact formula that was used to arrive at the end result (like at the beginning of the article with the kings question, with plugged in exact values from the specific problem at hand) - it would make it more of an education tool for people like me, who are trying to wrap their heads around this concept.

So instead of outputting a probability, a mathematical formula showing how to calculate the probability is displayed.