Implement the "status" command

Add a "status" console command. The initial implementation will list available tasks, and for each task display:

• Number of available source records (if the extractor is countable)
• Number of records which have been successfully migrated (if there is a KeyMap configured for the task)
• Number of unprocessed records (if both above numbers are available)
• Current status of the task (whether it is currently executing an operation, or is idle)