Desire to add "offset" language to ksy structure
Original issue https://github.com/kaitai-io/kaitai_struct/issues/1212 by @dhushon:

> Looking to parse "block" based binary structures...
> in these structures you might have fixed length blocks that have variable completion so the ksy might want to "read"/"write" the first say 256 bytes in a 1k block, and then skip(read) or pad(write) to the end of the block boundary.
> 
> Thoughts?