Provide a way to install additional drush commands during provision

[klonos]

Problem/Motivation

In order to support certain drush commands in Backdrop CMS, one needs to install https://github.com/backdrop-contrib/drush

Expected behaviour

Backdrop drush commands are installed OOTB.

Actual behaviour

There is no support for drush for Backdrop OOTB

Steps to reproduce / Proposed changes

These are the steps I take each time to add support for drush in Backdrop:

1. vagrant ssh
2. git clone https://github.com/backdrop-contrib/drush.git /usr/local/share/drush/commands/backdrop
3. drush cc drush

I wish that this was automated as much as possible.

PS: Thinking broadly here, it would greatly help if it was generally possible to specify additional drush commands to be downloaded from a git repo and placed inside /usr/local/share/drush/commands (then also run drush cc drush).