add INPUT_PULLDOWN support

Author: Links2004

cores/esp8266/Arduino.h

@@ -46,6 +46,7 @@ void yield(void);
 #define INPUT 0x0
 #define OUTPUT 0x1
 #define INPUT_PULLUP 0x2
+#define INPUT_PULLDOWN 0x3
 #define OUTPUT_OPEN_DRAIN 0x4
 
 #define PI 3.1415926535897932384626433832795

cores/esp8266/core_esp8266_wiring_digital.c

@@ -81,7 +81,12 @@ extern void __pinMode(uint8_t pin, uint8_t mode) {
         PIN_PULLUP_DIS(mux);
     } else if(mode == INPUT_PULLUP) {
         gpio_output_set(0, 0, 0, 1 << pin);
+        PIN_PULLDWN_DIS(mux);
         PIN_PULLUP_EN(mux);
+    } else if(mode == INPUT_PULLDOWN) {
+        gpio_output_set(0, 0, 0, 1 << pin);
+        PIN_PULLUP_DIS(mux);
+        PIN_PULLDWN_EN(mux);
     } else if(mode == OUTPUT) {
         gpio_output_set(0, 0, 1 << pin, 0);
     } else if(mode == OUTPUT_OPEN_DRAIN) {