Allow plugins to register custom feature parsers

[MeFisto94]

This is an interesting lifecycle question, because in order to be a plugin, you have to declare that via a feature parser, so at the time of loading the plugin, the features have to be parsed already.

Investigate what to do. Can we parse features later after plugins have been loaded, again? Or builtin vs extendable?
Another thought was to declare another feature like:

"andraste.builtin.featureprovider": {
   "featureParsers": [
       "myplugin.feature-one": "MyPlugin.FeatureOneParser"
   ]
}

While that would work, it still would have to duplicate information from the plugin feature or at least require to cross-reference that.
In addition it makes the parsing process still repeated, just transparent to the user. We need to parse this and the plugin feature at least, to be able to register and load all the plugins.

But then we're talking about a bit of json, we could probably just delay passing that with proper architecture around plugins