added tab completion for zsh
added tab completion for zsh

Changed to load tab completion based on available functions,
i.e. if `complete` exists setup bash autocompletions, if `compdef`
exists setup zsh completions.

Use `\ls -1 ...` in both functions, because `ls` sometimes gave me
improper results (trailing slashes etc.).

FYI, I don't like all the additional code being generated, but
can't really reuse bash completion code in zsh.

hmans/rbfu,hmans/rbfu