Handle more frequency options

[troygrosfield]

Would like to handle more frequency options like:

• "last day of the month"
• "every other week"
• "semi annual"
• "every other week"
• "first of the month"

This would require some additional handling which would something along the lines of:

if freq == Frequency.WEEKLY:
    kwargs['byweekday'] = [weekday_to_int(day)
                           for day in days_of_week] if days_of_week else None
elif freq == 'semi_annual':
    kwargs['freq'] = MONTHLY
    kwargs['interval'] = 6
elif freq == 'every_other_week':
    kwargs['freq'] = WEEKLY
    kwargs['interval'] = 2
elif freq == 'same_day_of_month':
    kwargs['freq'] = MONTHLY
    kwargs['bymonthday'] = 1
elif freq == 'first_day_of_month':
    kwargs['freq'] = MONTHLY
    kwargs['bymonthday'] = 1
elif freq == 'last_day_of_month':
    kwargs['freq'] = MONTHLY
    kwargs['bymonthday'] = -1