Add support for automatic sunset and sunrise detection via {suncalc}

[pat-s]

fixes #32

I also made some opinionated changes to the defaults of dark_start and dark_end to be able to add args lat and lon. This might break backward comp for some users even though I do not think that many will use the current defaults in production.

There might be more robust ways to implement this and as you can see I did not add any checks or assertions but it does the job ;)

I also did a run of styler::tidyverse_style() - just cherry pick the other changes if you dislike the styling.

[gadenbuie]

This is a really cool idea, thanks Pat! I've been picking away at a biggish internal refactor and I'll merge this when I get that worked out.