parse and to_string for Weekday and TimeOfDay

[DominikLutley]

Hey, I've been using this package and am finding it very useful.

I'm working a lot with TimeOfDay and Weekday and found myself writing a few helper functions, particularly parse_weekday(), week_day_to_string() and time_of_day_to_string().

Are these something you would be interested in adding to the package? I could work on this if you'd like.