Add support for specific dates (e.g., 9th of every month, June 9th, June 9th 2018 only)

[deoren]

For example, it would be useful to support these variations of June 9th (which is today, a Saturday):

• 2018_JUNE_09 or 2018_JUNE_9
• JUNE_09 or JUNE_9

It would also be useful to allow specifying loose days of the month that would apply to each month:

• 09 or 9

If there was an entry with that frequency it would fire today.

The existing weekly_saturday would also be lumped in, or if performance isn't a consideration, would be a separate query. Later when when support is added for start & repeat values we may have to rethink this slightly.

Example query that could work for this proposal:

SELECT * FROM `event_reminders`.`events` 
WHERE `event_schedule` in (
    '2018_JUNE_09', 
    '2018_JUNE_9',
    'JUNE_09',
    'JUNE_9',
    '09',
    '9',
    'weekly_saturday'
);

[deoren]

Note to self:

• https://pendulum.eustace.io/
• https://github.com/sdispater/pendulum

May not use the module, but it's worth researching.