Unify time modification on SD and SPIFFS (#738)

* Add access to last write date time

add example

* rename cpp to ino orz

* wrong copy -past

* No comment orz

* Add missing space

libraries/FS/src/FS.h

@@ -70,6 +70,7 @@ public:
     size_t size() const;
     void close();
     operator bool() const;
+    time_t getLastWrite();
     const char* name() const;
 
     boolean isDirectory(void);