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

This is the most annoying (to me) muscle-memory difference from grep.
 - `-F` matches POSIX [grep(1)](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/grep.html)
 - `--fixed-strings` matches GNU [grep(1)](https://www.gnu.org/software/grep/manual/grep.html#grep-Programs)
Just a minor nitpick: It'd be nice to add a `/* fall through */`here. I'll do it after merging your PR.