Make ConfigurationProperties public

This class is part of the public API (e.g. through the SerializerContext
 interface), so it should be accessible.

configlib-core/src/main/java/de/exlll/configlib/ConfigurationProperties.java

@@ -13,7 +13,7 @@
 /**
  * A collection of values used to configure the serialization of configurations.
  */
-class ConfigurationProperties {
+public class ConfigurationProperties {
     private final Map<Class<?>, Serializer<?, ?>> serializersByType;
     private final Map<Class<?>, Function<? super SerializerContext, ? extends Serializer<?, ?>>>
             serializerFactoriesByType;