Move reset, contrast, bias and extended commands to functions

In order to support using the same SPI bus for 3 displays or more and multiplexing the CE I need to reset all the displays but then send set up commands to each display in turn. So I have split things up a bit.

This does not change the default functionality but splits out the set up commands into their own functions.

* Fixed typo in the set_contrast command.
* Created a set_bias command.
* Created a extended_command function.
* Created a reset function.
* Moved the setup of the IO pins to the init function.
* Changed the begin to use new functions.