Add descriptor handling hook

Simple hook to allow user handling of descriptor requests (to support HID, etc). This allows, for example, fw/fw.c to do something like:

```c
void handle_get_descriptor() {
  switch ( SETUPDAT[3] ) {
    case DSCR_HID: //HID Descriptor
      SUDPTRH = MSB((WORD)&hid_dscr);
      SUDPTRL = LSB((WORD)&hid_dscr);
      return TRUE;
    case DSCR_HID_REPORT: //Report Descriptor
      //...
      return TRUE;
  }
  //did not handle the request
  return FALSE;
}
```