Don't turn on backlights during init.

[jzarl]

This avoids the short backlight flash when the backlight is immediately
turned back off (sometimes I only want to set the text without turning on the backlight).

[tdicola]

Thanks for submitting the pull and apologies it wasn't reviewed earlier. I just committed a change to add an optional backlight parameter to the initializer/constructor. The default value is ON which mimics the existing behavior of the library, but if you want to keep the backlight off when the display is turned on just pass in backlight=Adafruit_CharLCDPlate.OFF to the initializer. This way folks who have scripts that depend on the backlight automatically turning on won't be surprised if they update the library and it suddenly stops turning on the light. Thanks again for suggesting the change and submitting the pull!