Add new LogAscii::gzip_file_extension option.

This can be used with the LogAscii::gzip_level option to set the file extension of log files when they are compressed at creation time.

src/logging/writers/ascii/Ascii.h

@@ -64,6 +64,7 @@ private:
 	string meta_prefix;
 
 	int gzip_level; // level > 0 enables gzip compression
+	string gzip_file_extension;
 	bool use_json;
 	bool enable_utf_8;
 	string json_timestamps;