Feature: exclude paths during the backup process

__Proposed for merge__: tested on Magento 1.12 cluster.

# Changes

* Accepts array of paths to remove from the backup copy
* Aims to prevent accidental deletion of paths outside of the backup
* README updated for new feature

# Testing

* Give it a risky path: adding `/home` to the `paths_to_exclude` key does not recursively delete `/home`
* Set configurations that violate type expectations: empty array values are skipped