Add a --no-checksum option

[dscharrer]

For improved efficiency or to deal with broken installers.

Also, we should probably delete the broken and partial file(s) when checksums fail without this option.

[vv221]

While working on the GOG installer for Tohu, a ./play.it contributor encountered a Output checksum mismatch warning that seem to prevent the data extraction to complete: New game: Tohu - Work around an extraction error.

We are not sure yet if the checksum mismatch is the real cause of the interrupted extraction, but having a --no-checksum option (or a --keep-going one as suggested in #29) would help us investigate further.