Support InvalidCharHandler for reading

It looks like `InvalidCharHandler` can only be set when writing, but not reading. Do you think it makes sense to support this for reading as well? If a user is dealing with a `Reader` they could do this sort of transformation pretty easily before passing the stream to woodstox. However, it requires their code to understand which characters are invalid (rather than having woodstox be the source of truth for that). And if the user is dealing with an `InputStream`, they may not have an easy way to do character-based filtering/replacement