decoder_plugin: added method stream_tag()

This is like tag_dup(), but works with an input_stream object instead
of a file path.

test/read_tags.c

@@ -169,6 +169,20 @@ int main(int argc, char **argv)
 	}
 
 	tag = decoder_plugin_tag_dup(plugin, path);
+	if (tag == NULL && plugin->stream_tag != NULL) {
+		struct input_stream is;
+
+		if (!input_stream_open(&is, path, &error)) {
+			g_printerr("Failed to open %s: %s\n",
+				   path, error->message);
+			g_error_free(error);
+			return 1;
+		}
+
+		tag = decoder_plugin_stream_tag(plugin, &is);
+		input_stream_close(&is);
+	}
+
 	decoder_plugin_deinit_all();
 	input_stream_global_finish();
 	if (tag == NULL) {