added 'tasks' command

Hi Kohki,

This PR add the command "tasks" to list all defined tasks (without listing built-in commands):

```bash
altax tasks
```

You can also have the output formated in json this way:
```bash
altax tasks --format=json
```

It is usefull to integrate altax in another runner based on command line (my usecase).

I plan also to add support for `--format=json` to https://github.com/kohkimakimoto/altax/blob/master/src/Altax/Command/Builtin/NodesCommand.php

thanks,
Damien