Bash completion for the faas alias is not working

[mehyedes]

After installing faas-cli, a bash alias faas is also created by creating a symbolic link according to the PR #234
Bash completion does not work for the alias faas though, which forces you to resort to using faas-cli

Expected Behaviour

Bash completion works for both faas-cli and faas

Current Behaviour

Bash completion does not work for the alias faas.

Possible Solution

There is a helpful guide in the cobra repo:
https://github.com/spf13/cobra/blob/master/bash_completions.md#using-bash-aliases-for-commands
Creating a file under /etc/bash_completion.d/faas on Linux with the content below would fix this:

complete -o default -F __start_faas-cli faas

The completion works after sourcing that file

$ source /etc/bash_completion.d/faas
$ faas
auth        deploy      list        namespaces  secret      version   
build       describe    login       new         store                 
cloud       generate    logout      push        template              
completion  invoke      logs        remove      up

But that's just something I have tested quickly on Linux.

Steps to Reproduce (for bugs)

1. Install faas-cli
2. Try auto-complete with faas with Tab

Context

Creating the faas alias without bash completion makes the alias not very useful as it makes you resort to faas-cli most of the time to get completion for the commands.

Your Environment

• FaaS-CLI version ( Full output from: faas-cli version ):

faas-cli version 
  ___                   _____           ____
 / _ \ _ __   ___ _ __ |  ___|_ _  __ _/ ___|
| | | | '_ \ / _ \ '_ \| |_ / _` |/ _` \___ \
| |_| | |_) |  __/ | | |  _| (_| | (_| |___) |
 \___/| .__/ \___|_| |_|_|  \__,_|\__,_|____/
      |_|

CLI:
 commit:  2d183c713b32385831dc7f69c073e57c06e3b76c
 version: 0.12.2

• Docker version ( Full output from: docker version ):

docker version 
Client:
 Version:           19.03.6
 API version:       1.40
 Go version:        go1.12.17
 Git commit:        369ce74a3c
 Built:             Fri Feb 28 23:45:43 2020
 OS/Arch:           linux/amd64
 Experimental:      true

Server:
 Engine:
  Version:          19.03.6
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.12.17
  Git commit:       369ce74a3c
  Built:            Wed Feb 19 01:06:16 2020
  OS/Arch:          linux/amd64
  Experimental:     true
 containerd:
  Version:          1.3.3-0ubuntu1~18.04.2
  GitCommit:        
 runc:
  Version:          spec: 1.0.1-dev
  GitCommit:        
 docker-init:
  Version:          0.18.0
  GitCommit:

• Operating System and version (e.g. Linux, Windows, MacOS):
  Linux Ubuntu 18.04