Enable custom I2C interfaces

What's Changed

Enables using a non-default i2c / TwoWire port for boards that support multiple ports.

Added example LCD_Burnin.ino showing how to transfer a 20 char and 4 lines buffer to a device all at once.

New Contributors

• @uutzinger made their first contribution in #27

Full Changelog: 2.0.0...2.1.0