Support discarding empty blocks if flashing from .bz2 file

Hello,

Thanks for creating this tool! It would be nice if Etcher would support discarding empty blocks if flashing from .bz2 files. When I flash the .bz2 file it takes about 30 minutes to flash the image. If I extract the archive which contains a .sdimg file flashing only takes 5 minutes because Etcher is able to discard empty blocks in this case.

Best regards
Martin