Set start of agenda view

[niccokunzmann]

It would be nice to make the agenda more useful:

• I would like to display only what is today and in the future
• I would like to control how many events will be displayed.

See also: https://docs.dhtmlx.com/scheduler/agenda_view.html

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[niccokunzmann]

This has not been resolved by #158. But a start with the date parameter.