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/FSImpl.h

@@ -37,6 +37,7 @@ public:
     virtual size_t position() const = 0;
     virtual size_t size() const = 0;
     virtual void close() = 0;
+    virtual time_t getLastWrite() = 0;
     virtual const char* name() const = 0;
     virtual boolean isDirectory(void) = 0;
     virtual FileImplPtr openNextFile(const char* mode) = 0;