added next/previous weekday functionality

I have added two functions which return e.g. next Monday, second next Monday, previous Friday etc.

You could place `['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']` into a variable as you use it multiple times.

    weekdays = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
    if weekday not in weekdays:
        raise ValueError("Valid weekdays: '{}'".format("', '".join(weekdays)))
    weekday = weekdays.index(weekday)

Could also be done in `previous_weekday`.