Ignore unknown entries in plugin description files.

As #955 already describes if we use entries in the plugin description files (`plugin.yml`) that were implemented after the used BungeeCord version but are mostly optional (plugin works without it) we need to create 2 different JAR files because BungeeCord won't load plugins with unknown entries in the plugin description.
This can be fixed by modifying the SnakeYAML configuration a bit so it ignores them instead of throwing an error.

Maybe we should add a warning about unknown entries additionally? Currently this will just silently ignore them.