Added offset_date service - release candidate?

This is related to the manual_update feature. Check the 4.0.x release notes.

This is to simplify the automations, so that you do not have to do add/remove and keep converting detes from and to the date format.
Also updated the documentation and simplified the advanced example.

I'd call this a release candidate. Will give it a few days for testing.
As for the release, I keep thinking to mark the holiday_in_week_move or perhaps the whole holiday offset obsolete, create a separate entity for the holidays and handle that from the automation blueprints.

Please let me know your thoughts in the comments of #287.
It is a bit more work, but it gives the ultimate flexibility, and there are so many requests for different versions, exceptions, flavors (like if it moves to the weekend, move it somewhere else, etc).

Check out the README for examples.