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...