add support for INA239

[hgatesCPV]

Hi there, I would like to use this library to talk to INA229 and INA239. I have found in my application that SPI is much more resistant to being hung by transients than I2C. Any chance of making an option to use SPI instead of I2C? Thanks.

[hgatesCPV]

I would be happy to send you an eval board with this chip on it!