Use preexec hook to also index the files from the commnd line

[cristiansofronie]

Sometimes I remember the filename that I edited with vi or run some commands on
but not the directory. It would be pretty useful to also index that. The
Zsh completion system seems to know that is a file or we could just apply a
simple heuristic where we stat args.

If you think that this is useful I might give it a try soon.