RFC: Plugin discovery

I've had a look in to how external packages can register the fact that they contain PHPCI plugins and this is  a quick first draft of how it might work. Basically in the composer.json packages can add:

```yml
"extra": { 
	"phpci": {
		"pluginNamespace": "blahVendor/blahNs/",
		"suppliedPlugins": [
			{"class": "pluginClassNameOne", "name": "My first plugin"},
			{"class": "pluginClassNameTwo", "name": "My Second plugin"},
		]
	}
}
```

This information is then parsed by PHPCI for all installed packages and the list of available plugins is displayed.