fix issue #6 - suggest files

The new behavior is as follows:
in bash, whenever the next word can be an argument, files will be suggested along with the flags, regardless of the argument name (there are usually no subcommands to suggest when there are arguments so it's ok to suggest files)
in zsh, when the next word can be an argument: if the argument name contains "file" or "path", then files will be suggested, otherwise, as before, the name of the argument will be displayed