Introduce CLI completion to getenvoy

[musaprg]

Currently, we have to exec getenvoy fetch to know which version is available before executing getenvoy run. It is a small but actual pain for me. It would be nice to introduce CLI completion (bash-like or zsh-like) to getenvoy, especially getenvoy run. For instance, getenvoy can complete available "flavors" when typed "tab" after getenvoy run.

GetEnvoy uses https://github.com/spf13/cobra, which has a way to generate CLI completion for some popular shells (bash, zsh, fish, PowerShell) ¹. It could be done with it.

Footnotes

1. https://github.com/spf13/cobra/blob/master/shell_completions.md ↩

[codefromthecrypt]

I have personally found cobra very clunky. I wonder what you think about this? https://github.com/urfave/cli/blob/master/docs/v2/manual.md

[codefromthecrypt]

should be easy to get off cobra now

[codefromthecrypt]

@musaprg @llinder either of you want to try this after #218? here's an example project that bundles auto-completion https://github.com/rliebz/tusk

[codefromthecrypt]

I'm going to deprioritize this for now though someone else can grab it.

Note that auto-completion of built-in commands is usually easier and different than auto-completion of dynamic args (ex envoy versions). In each case, how to install is somewhat shell specific, so testing this well is a fair amount of work unless we punt and assume things just work.