Add Serial.clear() to Teensy 3.x

On Teensy 3.x - we have Serial1.clear, Serial2.clear...

But not Serial.clear.

Serial.clear() exists for Teensy2, so thought I would try to add it for
Teensy 3s...

teensy3/usb_serial.h

         virtual int read() { return usb_serial_getchar(); }
         virtual int peek() { return usb_serial_peekchar(); }
         virtual void flush() { usb_serial_flush_output(); }  // TODO: actually wait for data to leave USB...
+        virtual void clear(void) { usb_serial_flush_input(); }
         virtual size_t write(uint8_t c) { return usb_serial_putchar(c); }
         virtual size_t write(const uint8_t *buffer, size_t size) { return usb_serial_write(buffer, size); }
 	size_t write(unsigned long n) { return write((uint8_t)n); }