[core] json output for autodoc

Add json as output format for the B3 commands documentation feature.

The output looks like this:
```json
[
    {
        "description": "- register yourself as a basic user", 
        "plugin": "Admin", 
        "minlevel": 0, 
        "alias": "", 
        "maxlevel": 100, 
        "name": "!register"
    }
]
```

Here is an example of what you could do with it: my [B3 commands documentation](http://zeronode.de/b3doc.html)
In addition, I would like to commit the example in B3. What do you think?