src: Introduce a `-i` suffix flag for filter backups
src: Introduce a `-i` suffix flag for filter backups

This commit allows the user to chose to backup the files on which a
filter has been run, by specifying a suffix for the backup file. The
former implementation always backed up the files with a hardcoded
".kak-bak" suffix.

When no suffix is specified on the command line, the files are not
saved.

Fixes #1288

alexherbo2/kakoune,mawww/kakoune,casimir/kakoune,mawww/kakoune,danr/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,Somasis/kakoune,occivink/kakoune,occivink/kakoune,danr/kakoune,alexherbo2/kakoune,jkonecny12/kakoune,mawww/kakoune,danr/kakoune,lenormf/kakoune,occivink/kakoune,jjthrash/kakoune,casimir/kakoune,Somasis/kakoune,lenormf/kakoune,danr/kakoune,jjthrash/kakoune,jjthrash/kakoune,Somasis/kakoune,jkonecny12/kakoune,lenormf/kakoune,Somasis/kakoune,mawww/kakoune,casimir/kakoune,jjthrash/kakoune,casimir/kakoune,jkonecny12/kakoune,occivink/kakoune,lenormf/kakoune,alexherbo2/kakoune