On hook declaration, we can now add a help and/or a help_usage argument
to provide a simple way to the user to be informed.
For example:
```python
@hook("cmd_hook", "news", help_usage={"URL": "Display the latests news from a given URL"})
def cmd_news(msg):
[...]
```
will be displayed on !help !news as:
> Usage for command !news from module news: !news URL: Display the latests news from a given URL
Or for module commands help:
```python
@hook("cmd_hook", "news", help="display latests news")
def cmd_news(msg):
[...]
```
will be displayed on !help mymodule (assuming this hook is in the
module named mymodule) as:
> Available commands for module news: news: display latests news
Obviously, both `help` and `help_usage` can be present. If `help_usage`
doesn't exist, help on usage will display the content of help.
