Allow custom day formatter

[absynce]

Not sure if you're looking for contributions, but I am working through some changes made to fit my use case.

For this, I needed a custom day formatted because I wanted to show the relative offset from today below the day of the month:

That's why it expects a function with the zone, day, & today and returns Html Never:

dayFormatter :
    Time.Zone
    ->
        { day : Time.Posix
        , today : Time.Posix
        }
    -> Html Never

If you're not interested in this change, please feel free to decline.