code: static String getValue(final Map<String, Features> featureTable, final String extension) {
          final List<String> buf = new ArrayList<>();
          for (final Features f : featureTable.values()) {
              final List<String> v = f.getFeature(extension);
              if (v != null) {
                  buf.addAll(v);
              }
          }
          if (buf.isEmpty()) {
              return null;
          } else {
              return StringUtils.join(buf, ",");
          }
      }
docs: Get all and combine extension values
  
  @param featureTable plugin features
  @param extension extension ID
  @return combined extension value, {@code null} if no value available