I added tab completions for fish shell

[bagohart]

Hi, cool project! I added tab completions for fish shell:

complete sd -x -s f -l flags -d "<flags> Regex flags. May be combined (like `-f mc`)."
complete sd -x -s h -l help -d "Prints help information"
complete sd -s s -l string-mode -d "Treat expressions as non-regex strings"
complete sd -s p -l preview -d "Output result into stdout and do not modify files"
complete sd -x -s V -l version -d "Prints version information"

Do you want to bundle them with the project, or would you like me to create a PR to have them included in the fish shell repo instead?