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/joystick.h

@@ -0,0 +1,48 @@
+#ifndef JOYSTICK_H
+#define JOYSTICK_H
+
+#ifndef JOYSTICK_BUTTON_COUNT
+#define JOYSTICK_BUTTON_COUNT 8
+#endif
+
+#ifndef JOYSTICK_AXES_COUNT
+#define JOYSTICK_AXES_COUNT 4
+#endif
+
+#include <stdint.h>
+
+//configure on input_pin of the joystick_axes array entry to JS_VIRTUAL_AXIS 
+// to prevent it from being read from the ADC. This allows outputing forged axis value.
+//
+#define JS_VIRTUAL_AXIS 0xFF
+
+typedef struct {
+  uint8_t output_pin;
+  uint8_t input_pin;
+  
+  //the AVR ADC offers 10 bit precision, with significant bits on the higher part
+  uint16_t min_digit;
+  uint16_t max_digit;
+} joystick_config_t;
+
+extern joystick_config_t joystick_axes[JOYSTICK_AXES_COUNT];
+
+enum joystick_status{
+  JS_INITIALIZED = 1,
+  JS_UPDATED = 2
+};
+
+typedef struct {
+  
+  uint8_t buttons[JOYSTICK_BUTTON_COUNT/8+1];
+  
+  int8_t  axes[JOYSTICK_AXES_COUNT];
+  uint8_t status:2;
+} joystick_t;
+
+extern joystick_t joystick_status;
+
+//to be implemented in the hid protocol library
+void send_joystick_packet(joystick_t* joystick);
+
+#endif //JOYSTICK_H