Hide hints in dropdown to give people a chance to solve the challenge without them

[ostephens]

Suggested by @ldko in #196 : In challenges where there are hints, would it be better to do them as dropdowns to give people a chance to solve the challenge without them e.g.:

> > ## Hint
> > The `.` is also a metacharacter, so you will have to use the escape `\`
> > to express a literal period.