Improve auto-completion

[eduble]

The auto-completion can currently only complete the last word.
When returning to a previous word, auto-completion no longer works. This can be quite frequent when editing a command obtained from the history (e.g., by typing <ctrl-r> or using <up> arrow key).
It is probably possible to make this work.

[eduble]

It would also be better to warn when the completion file is outdated. Maybe by using an environment variable exported in the bash or zsh completion script, and the walt command could indicate this status when called without arguments for instance.
We would get one of those 3 lines:

shell completion: missing or outdated (= env variable not present)
shell completion: outdated (= env variable present and indicating an older version) -- <completion-file-path>
shell completion: up-to-date (= env variable present and indicating the current client version)

The completion file path could be obtained by using ${BASH_SOURCE[0]} for bash or ${(%):-%N} for zsh.