Add shorthand constants back to GPIO module and fix example

Author: vickash

README.md

@@ -21,6 +21,7 @@ In `CMakeLists.txt`, make sure the line `PRIV_REQUIRES` includes the `driver` co
 
 ## Example
 ```ruby
+include ESP32::Constants
 include ESP32::GPIO
 
 led = GPIO_NUM_4

mrblib/gpio.rb

@@ -2,6 +2,12 @@ module ESP32
   include Constants
 
   module GPIO
+    INPUT_PULLUP   = ESP32::GPIO_MODE_INPUT_PULLUP
+    INPUT_PULLDOWN = ESP32::GPIO_MODE_INPUT_PULLDOWN
+    INPUT          = ESP32::GPIO_MODE_INPUT
+    OUTPUT         = ESP32::GPIO_MODE_OUTPUT
+    INPUT_OUTPUT   = ESP32::GPIO_MODE_INPUT_OUTPUT
+    
     class << self
       alias :digital_write :digitalWrite   
       alias :digital_read  :digitalRead