Allow omitting boot and enable pins for Expander module

[falkoschindler]

In case users doesn't need to flash() or disconnect() an expander, they should be able to omit the boot and enable parameters:

expander = Expander(serial, [boot, enable])

[JensOgorek]

I will have a look at this.

[JensOgorek]

I created a pull request (#55) with the modified expander