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.
We generally don't seem to document WrongType (with a few exceptions).  Is just having `@throws WrongType` (as in `syscall.vim`) useful, or do we want e.g. `@throws WrongType if {F} is not a function or [fire_immediately] is not a number.`?