Implemented a weekdayToNumberConverter. Test Passing.

Let me know if there is a better place to include these little utilities in the code base, or if this one needs improvement.
`weekdayToNumberConverter` suggests that this may return some sort of _converter_ object.

Perhaps we could rename this to something that feels more actionable. Maybe using `parse` or `convert`. You choose… `parseWeekday`, `convertWeekdayToInt`, or something similar.