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.
Yeah, I think having `@throws WrongType` is reasonable since it's self-explanatory why WrongType would be thrown, and the reason tends to get stale if it mentions each argument it checks. The type constraints should be mentioned in the param documentation whenever there's any ambiguity, though.

The reason it helps having a simple `@throws WrongType` even though it's trivially obvious from the function body is that it might be deep in some other call stack and you shouldn't have to crawl deep into the call stack to find all the errors thrown.