add support for hid gamepad interface

add documentation for HID joystick
Add joystick_task to read analog axes values even when no key is pressed or release. update doc
Update docs/feature_joystick.md
Manage pin setup and read to maintain matrix scan after analog read

quantum/process_keycode/process_joystick.c

@@ -0,0 +1,79 @@
+#include "process_joystick.h"
+
+#include <quantum/joystick.h>
+#include <quantum/quantum_keycodes.h>
+
+#ifdef __AVR__
+# include <drivers/avr/analog.h>
+#endif
+
+#include <string.h>
+
+bool process_joystick_buttons(uint16_t keycode, keyrecord_t *record);
+
+bool process_joystick(uint16_t keycode, keyrecord_t *record){
+  
+  if (process_joystick_buttons(keycode, record) 
+    && (joystick_status.status & JS_UPDATED)>0){
+    send_joystick_packet(&joystick_status);
+    joystick_status.status &= ~JS_UPDATED;
+  }
+  
+  return true;
+}
+
+__attribute__ ((weak))
+void joystick_task(void){
+  if (process_joystick_analog() && (joystick_status.status & JS_UPDATED)){
+    send_joystick_packet(&joystick_status);
+    joystick_status.status &= ~JS_UPDATED;
+  }
+}
+
+bool process_joystick_buttons(uint16_t keycode, keyrecord_t *record){
+  
+  if (keycode < JS_BUTTON0 || keycode > JS_BUTTON_MAX){
+    return true;
+  } else {
+    if (record->event.pressed){
+      joystick_status.buttons[(keycode-JS_BUTTON0)/8] |= 1<<(keycode%8);
+    } else {
+      joystick_status.buttons[(keycode-JS_BUTTON0)/8] &= ~(1<<(keycode%8));
+    }
+    
+    joystick_status.status |= JS_UPDATED;
+  }
+  
+  return true;
+}
+
+__attribute__ ((weak))
+bool process_joystick_analog(){
+#if JOYSTICK_AXES_COUNT > 0
+  for (int axis_index=0 ; axis_index<JOYSTICK_AXES_COUNT ; ++axis_index){
+    if (joystick_axes[axis_index].output_pin==JS_VIRTUAL_AXIS || joystick_axes[axis_index].input_pin==JS_VIRTUAL_AXIS){
+      continue;
+    }
+    
+    setPinOutput(joystick_axes[axis_index].output_pin);
+    writePinHigh(joystick_axes[axis_index].output_pin);
+    
+    //disable pull-up resistance
+    setPinInput(joystick_axes[axis_index].input_pin);
+    writePinLow(joystick_axes[axis_index].input_pin);
+    
+#ifdef __AVR__
+    int16_t axis_val = analogReadPin(joystick_axes[axis_index].input_pin);
+#else
+    int16_t axis_val = 0;
+#endif
+    if (axis_val!=joystick_status.axes[axis_index]){
+      joystick_status.axes[axis_index] = axis_val;
+      joystick_status.status |= JS_UPDATED;
+    }
+    
+    writePinLow(joystick_axes[axis_index].output_pin);
+  }
+#endif
+  return true;
+}