[Feature] Shell completions

[iwilltry42]

This PR aims to add/improve the shell completions provided by k3d by implementing custom functions.
Since our usage strings are a bit special, at least for zsh we need to use a fork of cobra until spf13/cobra#1070 got merged.

[marckhouzam]

Cool! I actually use k3d to test spf13/cobra#1070 (and many other things). Thanks @iwilltry42 for improving its completion!

[iwilltry42]

@marckhouzam that's super cool :)
Thanks a lot for working on the cobra completion stuff :)
Unfortunately, something is broken in the released binaries of k3d.. it works building them locally though

[marckhouzam]

@iwilltry42 broken with completion? I'm curious to see what is happening in case it's a big in Cobra. Can you clarify?

[iwilltry42]

@marckhouzam I doubt it's a bug in Cobra 👍
The "broken" thing here was, that shell completion would fail with the issues that your PR fix (in this case it's escaping the brackets in command descriptions) in the builds created by Travis CI while they all work well when I build them on my machine ("Works on my machine", you know? 😬).
So I assume that it's an issue with managing dependencies 🤔