patch 7.4.1578

Problem:    There is no way to invoke a function later or periodically.
Solution:   Add timer support.

src/feature.h

@@ -399,6 +399,13 @@
 # define FEAT_RELTIME
 #endif
 
+/*
+ * +timers		timer_start()
+ */
+#if defined(FEAT_RELTIME) && (defined(UNIX) || defined(WIN32))
+# define FEAT_TIMERS
+#endif
+
 /*
  * +textobjects		Text objects: "vaw", "das", etc.
  */