Add `-k` `--keep` to be compatible with other zip tools

There are a few things that are ubiquitous across `z`-ing CLIs: ```text -d --decompress force decompression -k --keep keep (don't delete) input files -1 ... -9 compression preset -q --quiet suppress noncritical error messages -v --verbose be verbose (a 2nd -v gives more) -c --stdout write to stdout, keep original files -V --version display program version -t, --test test compressed file integrity ``` It would be a boon to the world at large if you were to, as much as possible, add compatibility with the flags that are already shared among `z`-ing tools, and in the muscle-memory of unixers worldwide. 🙂 ## `xz --help` ```text -z, --compress force compression -d, --decompress force decompression -t, --test test compressed file integrity -l, --list list information about .xz files -k, --keep keep (don't delete) input files -f, --force force overwrite of output file and (de)compress links -c, --stdout write to standard output and don't delete input files -0 ... -9 compression preset; default is 6; take compressor *and* decompressor memory usage into account before using 7-9! -e, --extreme try to improve compression ratio by using more CPU time; does not affect decompressor memory requirements -T, --threads=NUM use at most NUM threads; the default is 1; set to 0 to use as many threads as there are processor cores -q, --quiet suppress warnings; specify twice to suppress errors too -v, --verbose be verbose; specify twice for even more verbose -h, --help display this short help and exit -H, --long-help display the long help (lists also the advanced options) -V, --version display the version number and exit ``` ## `gzip --help` ```text -1 --fast fastest (worst) compression -2 .. -8 set compression level -9 --best best (slowest) compression -c --stdout write to stdout, keep original files --to-stdout -d --decompress uncompress files --uncompress -f --force force overwriting & compress links -h --help display this help -k --keep don't delete input files during operation -l --list list compressed file contents -N --name save or restore original file name and time stamp -n --no-name don't save original file name or time stamp -q --quiet output no warnings -r --recursive recursively compress files in directories -S .suf use suffix .suf instead of .gz --suffix .suf -t --test test compressed file -V --version display program version -v --verbose print extra statistics ``` ## bzip2 --help ```text -h --help print this message -d --decompress force decompression -z --compress force compression -k --keep keep (don't delete) input files -f --force overwrite existing output files -t --test test compressed file integrity -c --stdout output to standard out -q --quiet suppress noncritical error messages -v --verbose be verbose (a 2nd -v gives more) -L --license display software version & license -V --version display software version & license -s --small use less memory (at most 2500k) -1 .. -9 set block size to 100k .. 900k --fast alias for -1 --best alias for -9 ```