Support for date-specific times

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.

Start by reading the parser and the existing date.register token behavior described in the issue, then review SunCalc for the sunrise and sunset dependency. Done should include a way to register callbacks that receive the determined date and return a time, supporting location-aware tokens such as sunrise and sunset without breaking existing tokens.