List the available_tasks

def list_tasks(collector, tasks, **kwargs):
    """List the available_tasks"""
    print("Available tasks to choose from are:")
    print("Use the --task option to choose one")
    print("")
    keygetter = lambda item: item[1].label
    tasks = sorted(tasks.items(), key=keygetter)
    for label, items in itertools.groupby(tasks, keygetter):
        print("--- {0}".format(label))
        print("----{0}".format("-" * len(label)))
        sorted_tasks = sorted(list(items), key=lambda item: len(item[0]))
        max_length = max(len(name) for name, _ in sorted_tasks)
        for key, task in sorted_tasks:
            desc = dedent(task.description or "").strip().split('\n')[0]
            print("\t{0}{1} :-: {2}".format(" " * (max_length-len(key)), key, desc))
        print("")