Add WP CLI command (Update)

[graham73may]

Sorry for the new PR, I couldn't contribute to the original PR and wanted to contribute in a way that provided a mergeable update.

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I've taken a look at BramDriesen's work in #24 but ran in to a couple of issues.

1. Renamed the command for consistency with core (commit)

wp batch-processing {enter_your_batch_id}

2. Function get_all_items() doesn't seem to exist in /includes/class-batch.php. I have made $items public and adjusted class-wp-cli.php accordingly. (commit)

I've opted for making $items public instead of adding the missing get_all_items() function, I wasn't sure if this was missing from the original PR or was removed from the main plugin at some point. Making the property public seemed to be a low risk option with minimal code changes.

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I believe this PR is mergeable, but having used it some QoL additions would be to include a command to:

1. List available batches
2. Restart a batch

[graham73may]

I've added the subcommands I mentioned in my original message.

Running wp help batch-process now gives:

NAME

  wp batch-process

DESCRIPTION

  Class Batch_Processor_CLI_Command().

SYNOPSIS

  wp batch-process <command>

SUBCOMMANDS

  list         List all registered batches.
  process      Process a registered batch by ID.
  restart      Restart a registered batch by ID.

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wp batch-process process batch_name_one

Starting processing batch: batch_name_one with 6 items
Processing batch items  100% [=================================================================================================================================================================================] 0:03 / 0:03

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wp batch-process list

wp batch-process list
+--------------------+-------+-----------+------------+----------+
| ID                 | Total | Processed | Percentage | Finished |
+--------------------+-------+-----------+------------+----------+
| batch_name_one     | 6     | 6         | 100.00%    | Yes      |
| batch_name_two     | 0     | 0         | 0.00%      | Yes      |
+--------------------+-------+-----------+------------+----------+

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wp batch-process restart {batch_id_here}

wp batch-process restart batch_name_one
Success: Batch 'batch_name_one' restarted.