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

[AJ ONeal]

There are a few things that are ubiquitous across z-ing CLIs:

   -T, --threads=NUM   use at most NUM threads; the default is 1; set to 0
   -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. 🙇‍♂️ 🙂

(and, of course, the standard POSIX, BSD, GNU, and IEEE command line flags)

xz --help

  -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

 -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

   -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