Add mechanism for posting functions to the main loop (#2082)

* Add mechanism for posting functions to the main loop (#1064)

* Fix indentation, add note that API is not stable

cores/esp8266/Schedule.h

@@ -0,0 +1,27 @@
+#ifndef ESP_SCHEDULE_H
+#define ESP_SCHEDULE_H
+
+#include <functional>
+
+#define SCHEDULED_FN_MAX_COUNT 32
+#define SCHEDULED_FN_INITIAL_COUNT 4
+
+// Warning 
+// This API is not considered stable. 
+// Function signatures will change.
+// You have been warned.
+
+// Run given function next time `loop` function returns, 
+// or `run_scheduled_functions` is called.
+// Use std::bind to pass arguments to a function, or call a class member function.
+// Note: there is no mechanism for cancelling scheduled functions.
+// Keep that in mind when binding functions to objects which may have short lifetime.
+// Returns false if the number of scheduled functions exceeds SCHEDULED_FN_MAX_COUNT.
+bool schedule_function(std::function<void(void)> fn);
+
+// Run all scheduled functions. 
+// Use this function if your are not using `loop`, or `loop` does not return
+// on a regular basis.
+void run_scheduled_functions();
+
+#endif //ESP_SCHEDULE_H