Additional Configuration/Options for Debug Process Plugin

Patch attached.

This needs documentation in the readme.

This example is missing the source key and should use the same example field name as the other examples. And maybe use another options as example a "source" may be confusing. It should be something like:

* @code
 * process:
 *   field_tricky:
 *     -
 *       plugin: debug
 *       dump: source_ids
 *.      source: whatever
 * @endcode
 *
 * This will print all IDs from the source. Options for the "dump"
 * configuration are:

Rerolled patch with updated documentation. The functionality lgtm as well.

Thanks for helping out with this one

Automatically closed - issue fixed for 2 weeks with no activity.