Add a FLAG.AddCallback variant that doesn't fire immediately (#95)

Adds an optional `fire_immediately` parameter to `Flag.AddCallback` to allow the initial call to be suppressed.

Also fixed some spelling mistakes in the Vroom test comments, and regenerated the documentation.
Probably want maktaba#ensure#IsNumber here.

Also looks like this and m#function#Apply() are missing @throws WrongType (along with a bunch of other functions here, I'm sure).