Add a dry-run option

Problem/Motivation

Will be great if we have an option to see what will be happens once the cron run, something like the

Test run now without deleting and output result

option added in https://www.drupal.org/node/1786720 in the patch #17.

Proposed resolution

Add a option in the module config page to made a Dry run:

[] Dry Run
Test run without deleting revisions but seeing the output results.

This option only will be visible if you have checked the Delete revisions now checkbox.

Remaining tasks

None.

User interface changes

None.

API changes

A new option added in the module config page.

Data model changes

None.