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.cpp

@@ -32,6 +32,15 @@ size_t File::write(uint8_t c)
     return _p->write(&c, 1);
 }
 
+time_t File::getLastWrite()
+{
+    if (!_p) {
+        return 0;
+    }
+
+    return _p->getLastWrite();
+}
+
 size_t File::write(const uint8_t *buf, size_t size)
 {
     if (!_p) {