Added support of generic streams instead of hardcoded software serial.

[vladkozlov69]

Hello Tobias,

I've refactored your code a bit to make it working with any Stream& and not only SoftwareSerial (tested using STM32 HardwareSerial and SC16IS750 I2C-to-UART board).
Decided to share my changes with you.

[tobiasschuerg]

Thanks a lot. I'll test with my setup once got time an merge. 👍

[vladkozlov69]

Just in case here is a snippet from my sketch:

`#include <Wire.h>
#include <SC16IS750.h>
#include <MHZ.h>

#define MHZ_PWM_PIN PC13

SC16IS750 i2cuart = SC16IS750(SC16IS750_PROTOCOL_I2C, SC16IS750_ADDRESS_AA);
MHZ co2(&i2cuart, MHZ_PWM_PIN, MHZ19B);

void setup()
{
Wire.begin();
i2cuart.begin(9600);
}`