Allow for looping to be used for multiple commands.

[jnettik]

I recently needed this plugin as well as similar functionality for other commands. I copied the approach for looping over sites into a Trait for my own plugin, and was able to apply the same logic for Drush and the backup creation commands. Before publishing that plugin, I wanted to see if there was room in this project to share the looping logic and be able to implement more Terminus commands over time. My plugin is at https://github.com/jnettik/terminus-mass-run