Add more riddles!

This package ships with 3 riddles now, but i would be great to have a wider selection. If you're interested in contributing, take a look at some of the [existing riddles](https://github.com/dgkf/riddlr/tree/master/inst/example/questions). In particular, [Riddle \#1](https://github.com/dgkf/riddlr/blob/master/inst/example/questions/q1.riddlr.Rmd) has notes to a new riddle author about how to modify this file as a template to develop a new riddle.

For this public release, I'd prefer if questions are relatively generic with minimal field-specific terminology. Also remember that users may choose to approach problems in a number of different ways, so it's advised to keep your expected solution output as generic as possible. In most cases, this means that you want to produce a simple value or vector, avoiding issues with package classes and object attributes.