Support shortcode arguments for booking at set dates/times

[NateWr]

The shortcode should allow date and/or time values to be passed in to display a booking form with a set date/time. This would allow restaurants to offer special booking forms for holidays or other events.

This needs some further thought. If the goal is to allow a form with a fixed date, there may need to be the ability to pass time options for multiple sittings in a day. Maybe the ability to pass multiple dates too, to turn it into a select field.

Alternatively, if it's just a recommended date/time, the date/time pickers should still show, just with pre-selected values.

Two different use-cases that may need different approaches.

[NateWr]

A similar use-case would be the ability of someone who's booking has been rejected to be able to click a link and have the form pre-populated with their name/email/party/phone/message for a new booking.