Pass position in input to `begin_game`

[parszab]

Thanks for the library, it is fantastic!

However, to implement a tool that would require the position of a particular PNG in the file it would be necessary to know the byte position of each game in the input.

(For pgn-extract like functionality e.g. to get the PGNs that match a search or to return the position of the invalid PGNs in a file.)

Would it be possible to pass the input position in begin_game?