add possibility to override config via system properties

src/main/java/jpass/util/Configuration.java

@@ -31,12 +31,18 @@
 import java.io.File;
 import java.io.FileInputStream;
 import java.io.InputStream;
+import java.util.Optional;
 import java.util.Properties;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
 /**
- * Class for loading configurations from {@code jpass.properties}.
+ * Class for loading configurations from {@code jpass.properties} or system properties.
+ *
+ * <p>
+ * Each configuration property can by overridden by system properties,
+ * with the {@code jpass.} prefix.
+ * </p>
  *
  * @author Gabor_Bata
  */
@@ -75,7 +81,8 @@ private File getConfigurationFolderPath() {
 
     private <T> T getValue(String key, T defaultValue, Class<T> type) {
         T value = defaultValue;
-        String prop = properties.getProperty(key);
+        Optional<String> systemProp = Optional.ofNullable(System.getProperty(String.format("jpass.%s", key)));
+        String prop = systemProp.orElseGet(() -> properties.getProperty(key));
         if (prop != null) {
             try {
                 value = type.getConstructor(String.class).newInstance(prop);