Issue title: Idea: Shift Register Implementation by inheritance
 Issue body: Hi Dean,

i'm very fond of your library. It's great.
However when using the Display lots of port are getting blocked (12)

Using a Shift Register is moving the issue to external shift registers.
with this only 3! ports are used by this display.

I've created an child class from your parent,
with least possible intrusion and maximum possible re-usage of your code.

What do you think?
Could this be integrated into your library?

Jens
 Comments: 
 Comment 0: i'll keep the changes in the fork...
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