add online-recognizer (#29)

sherpa-onnx/csrc/features.h

@@ -6,6 +6,7 @@
 #define SHERPA_ONNX_CSRC_FEATURES_H_
 
 #include <memory>
+#include <string>
 #include <vector>
 
 namespace sherpa_onnx {
@@ -13,6 +14,8 @@ namespace sherpa_onnx {
 struct FeatureExtractorConfig {
   float sampling_rate = 16000;
   int32_t feature_dim = 80;
+
+  std::string ToString() const;
 };
 
 class FeatureExtractor {