Add auto write?

[caternuson]

Would it make sense to add a toggle-able auto-write capability, similar to neopixel. That way user doesn't have to always call show() to update the display and see what they sent it. I think having this on by default would be more beginner friendly, so they could just:

disp.print("BEEF")

and it would show up right away.

It could be turned off, reverting to current behavior, for more advanced usage.

[tannewt]

Yup, I like autowrite.

[sommersoft]

Completed by #14.