Add a resume mechanism

[jigarius]

Motivation

• Say, you have a Drupal installation with a 100 sites.
• Say, Drall stops abruptly after 30 sites.
• There should be a way to resume the Drall operation without having to repeat the 30 sites that are already done.

Proposed solution

• Add an option --batch-file=/path/to/batch.json to track progress.
• When using resume, store Drall's execution state in a batch.json.
• If a batch file already exists, then resume.

[jigarius]

In addition to tracking progress, the batch file also provides information about:

• The state of each item.
• The start and finish time for each item.
• The time taken for each item.
• The exit code for each item.

The FileBatch class allows reading this information. It should be possible to use this class to build reports.