Support for date-specific times

[safinea]

I would love for this library to support tokens that calculate the time. To be concrete, it would be brilliant if I could, for example, get the sunrise time for a particular date, or the sunset time (e.g. 'next week monday at sunrise'). This depends on a) the date and b) the location. (See also https://github.com/mourner/suncalc) Another example could be 'wakeuptime' that's set to 7:00 during the week and 10:00 in the weekends.

I can imagine something along the lines of registering a token (e.g. 'sunset') with the parser with a callback function that gets passed the determined date and returns the appropriate time.

Easy example (registering 'midnight' as a token):

date.register('midnight', function(d) { return d.setHours(0, 0, 0) });

I know 'midnight' already exists, but it's just an example. This feature could also make 'evening', 'night', and so on more dynamic and user customizable.

Thanks.

[matthewmueller]

+1, accepting PRs :-)