Added a rudimentary timer implementation

includes/timer.h

@@ -0,0 +1,35 @@
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* This file defines a simple timer struct, and methods to initialize it,
+and keep track of time elapsed.
+It was copied from https://github.com/raysan5/raylib/wiki/Frequently-Asked-Questions#how-do-i-make-a-timer */
+
+typedef struct Timer {
+    double start_time;   // Start time (seconds)
+    double lifetime;    // Lifetime (seconds)
+} Timer;
+
+Timer timer_init(double lifetime_secs)
+{
+    Timer timer;
+    timer.start_time = GetTime();
+    timer.lifetime = lifetime_secs;
+    
+    return timer;
+}
+
+bool timer_done(Timer timer)
+{
+    return GetTime() - timer.start_time >= timer.lifetime;
+}
+
+double timer_get_elapsed(Timer timer)
+{
+    return GetTime() - timer.start_time;
+}
+
+#ifdef __cplusplus
+}
+#endif