toast.promise loading as ValueOrFunction<Renderable, T>

[TastyDucks]

The current API for toast.promise allows functions to return a value for success or error messages -- it would be nice to be able to do the same for the loading message.

Example use case: updating the progress of an upload, e.g:

loading: () => prog !== -1 ? `Slicing toast: ${prog}%` : "Toasting..."
success: "🍞🧈🏁"
error: "Toast was burnt 😞"

This would change the Type signature from

loading: Renderable

to

loading: ValueOrFunction<Renderable, any> // In the same manner as success and error.