add pio example for 7_segment display

[mjcross]

Add an example showing how to use the PIO to control a multiplexed 7-segment display.

[lurch]

Replaces #372

[mjcross]

HI guys - is there some reason this one isn't progressing?

[mjcross]

Bump

[peterharperuk]

No. It'll be considered for the next sdk release.

[mjcross]

Thanks - I think it's a useful example because it shows how to keep outputting the same pattern until something arrives on the FIFO