Add "overflow()" to HardwareSerial

Hi
it would be nice, to have an overflow handler in the HardwareSerial class. This would be handy if you have code (i.e. rendering a display) where you can not fetch the data and the serial buffer would overflow otherwise...
This could be simply achieved by
```c++
//HardwareSerial.cpp line 92

bool handle_serial_overflow() **attribute**((weak));
bool handle_serial_overflow(){ return false}

inline void store_char(unsigned char c, ring_buffer *buffer)
{
int i = (unsigned int)(buffer->head + 1) % SERIAL_BUFFER_SIZE;

if (i != buffer->tail) {
buffer->buffer[buffer->head] = c;
buffer->head = i;
}

//*******************\* overflow handling *******************************
else if( handle_serial_overflow()) {
store_char(c, buffer);
}
//************************************************************
}
```