Allow manual rotation of rotating_file_sink (#3269)

* Allow manual rotation of rotating_file_sink

* Rename rotation method

* Attempted fix for tests on Windows

* Apply review mark-ups

include/spdlog/sinks/rotating_file_sink.h

@@ -27,6 +27,7 @@ public:
 
     static filename_t calc_filename(const filename_t &filename, std::size_t index);
     filename_t filename();
+    void rotate_now();
 
 protected:
     void sink_it_(const details::log_msg &msg) override;