#11 Compression support

[Dom De Vitto]

The ability to write compressed output (gzip/bzip/whatever) would be really useful. e.g. compression=-9. Even if this was implemented as a child process command pipethrough="gzip -9" this would save time & space, which with large compressible images, would be a great improvement. I appreciate you can achieve this with of=- | gzip -9, but as a shell pipe, dc3dd is unaware if the pipe fails, leaving the final file truncated, requiring decompression & rehashing to ensure the file is on disk correctly.

Basically, dc3dd of=- | gzip -9 && echo Compressed file written correctly.

Arguably, running a gzip --test on the final output file, reading from the output file in the same loop as the writes, would be very effecient in time/resource, and give an absolute that the written file was not corrupt.