Extend 'set_blink_rate()' to turn off display with value of '-1'

[mungewell]

I have a project with multiple modules, when 'blink()' was enabled they blinked a slightly different rates... which looked funky.

The register for blinking also includes a 'display_on' bit, so I access that with a vaule of '-1' to turn off the display(s). Set a normal blink rate to turn display(s) back on.

Tested on the 'HT16K33Segment' from Adafruit.

[mungewell]

I realized that it would be more useful if repeated writes of '-1' toggled the display off, on, off, on. That way the app wouldn't need to track the state.

I'll revise the patch tonight...

[smittytone]

I'm going to hold off on this PR and look at other ways to implement this functionality. I don't think set_blink_rate() should be used as a proxy for turning the display on or off, as that's a side effect not indicated by the function's name. For example, I'd rather add display_on()/display_off functions, which make what they do explicit.

[smittytone]

See the develop branch for how it's looking