Fix shell completions output directory

[orhun]

I realized the shell completions generation was added in the last release so I have it a try for the Arch package:

$ atac completions -h

atac completions [OPTIONS] <SHELL> [OUTPUT_DIRECTORY]

$ atach completions bash target

error: unexpected argument '--directory' found

Uhh, what?

It turns out --directory argument was necessary for all cases so the correct command would be:

$ atac completions --directory target completions bash

I don't find this intuitive - and it clashes with the help message. So I fixed this in this PR and made the following possible:

$ atac completions bash target

Completions file generated into "target"

Also, it shows an error if you don't provide any directories as the trailing argument. (We can probably improve the error message there so LMK)

[Julien-cpsn]

Closed by https://github.com/Julien-cpsn/ATAC/releases/tag/v0.18.0