Made select pin adjustable

[itavero]

Adjusted the begin method. It now accepts a single argument, indicating the Slave Select / Chip Select pin.
It defaults to 10, so it should be backwards compatible.

[zachwick]

I think that instead of making the CS pin be an argument passed to the ADXL362::begin method, it should instead be an argument passed to the ADXL362::ADXL362 constructor.
I have implemented such a feature (and making the debug setting a per object option) in my "updates" branch which can be found at https://github.com/ZachWick/ADXL362/tree/updates.
I haven't submitted a pull request for these changes yet however because I am waiting to see if @annem will accept my PR ( #4 ) to change the licensing to GNU GPLv3 before I submit more changes.

[itavero]

Implemented in 640ecd2.