Add completion for fish shell

[mamyn0va]

Currently, only bash and zsh completions are supported by cozy-stack CLI. As fish shell is getting more and more popular among user-friendly shells, I think it deserves its own completion.

[nono]

We are using a library called cobra for managing the CLI options parsing. There is a PR opened for adding fish completion, but it has not been merged yet: spf13/cobra#754

[mamyn0va]

Is this independent from cozy-stack ? I mean, as soon as cobra will merge this PR, will the fish completion be working in cozy-stack, or will you have to create a completion file, as for bash and zsh ?

[nono]

When the PR will be merged, we will update https://github.com/cozy/cozy-stack/blob/master/cmd/completion.go, and use it to generate the completion file for fish.

[mamyn0va]

Ok, great.