add -F/--fixed-strings as alias for -Q/--literal

-F is compatible with POSIX grep(1)
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/grep.html

--fixed-strings is compatible with GNU grep(1)
https://www.gnu.org/software/grep/manual/grep.html#grep-Programs

This is the most annoying (to me) muscle-memory difference from grep.
document -F/--fixed-strings
add bash completion for -F/--fixed-strings