change library so it can apply to the iic bus other than the default one

[leoncoolmoon]

Thank you for creating a pull request to contribute to Adafruit's GitHub code!
Before you open the request please review the following guidelines and tips to
help it be more easily integrated:

• change the begin() to add the IIC pin define so you can use other pin as the IIC for this library

• limited test

• the original code still works, other than that on ESP32 you can use pin 14 as I2C_SCL and pin 15 as I2C_SDA define an iic bus and run the code as well

Thank you again for contributing! We will try to test and integrate the change
as soon as we can, but be aware we have many GitHub repositories to manage and
can't immediately respond to every request. There is no need to bump or check in
on a pull request (it will clutter the discussion of the request).

Also don't be worried if the request is closed or not integrated--sometimes the
priorities of Adafruit's GitHub code (education, ease of use) might not match the
priorities of the pull request. Don't fret, the open source community thrives on
forks and GitHub makes it easy to keep your changes in a forked repo.

After reviewing the guidelines above you can delete this text from the pull request.