Add a new 'task list' cli command

• Add a SchedulerBackend.search_tasks method

• Add a 'full' flag argument to the pretty_print_task function

  to display also the status and priority fields.

• Add a 'task list' cli command to list tasks with search criterions

  and not only pending tasks.

Depends on !39 (closed)

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Migrated from D934 (view on Phabricator)