Prints task information using io stream.

        `env`
            ``Environment`` object.
        `tasks`
            List of tuples (task_name, options, block_options).
        `mark_active`
            Set to ``True`` to mark active task.

def _print_tasks(env, tasks, mark_active=False):
    """ Prints task information using io stream.

        `env`
            ``Environment`` object.
        `tasks`
            List of tuples (task_name, options, block_options).
        `mark_active`
            Set to ``True`` to mark active task.
        """

    if env.task.active and mark_active:
        active_task = env.task.name
    else:
        active_task = None

    for task, options, blocks in tasks:
        # print heading
        invalid = False

        if task == active_task:
            method = 'success'
        else:
            if options is None and blocks is None:
                method = 'error'
                invalid = True

            else:
                method = 'write'

        opts = list(options or [])
        blks = list(blocks or [])

        write = getattr(env.io, method)
        write('~' * 80)
        write(' ' + task)
        write('~' * 80)
        env.io.write('')

        # non-block options
        if opts:
            for opt, values in opts:
                env.io.write('    {0}: {1}'.format(opt,
                             ', '.join(str(v) for v in values)))
            env.io.write('')

        # block options
        if blks:
            had_options = False

            for block, options in blks:
                if options:
                    had_options = True
                    env.io.write('    {{ {0} }}'.format(block))

                    for opt, values in options:
                        env.io.write('        {0}: {1}'.format(opt,
                                     ', '.join(str(v) for v in values)))
                    env.io.write('')

            if not had_options:
                blks = None

        if not opts and not blks:
            if invalid:
                env.io.write('  Invalid task.')
            else:
                env.io.write('  Empty task.')
            env.io.write('')