Add command to generate Bash autocomplete script

                   
|Q            |A  |
|---          |---|
|Bug Fix?     |n  |
|New Feature? |Y  |
|BC Breaks?   |n  |
|Deprecations?|n  |
|Tests Pass?  |Y  |
|Fixed Tickets|   |
|License      |MIT|
|Doc PR       |   |
                   

Add command to generate script for Bash autocompletion. This script allows autocomplete support when typing "gush" followed by a [tab], which gives you all the available commands, as well as the options for the commands

Sent using [Gush](https://github.com/gushphp/gush)