Feature request: ability to control the brightness value at which a color is considerd dark or light

[DanielCashlink]

Currently the isDark and isLight is determined by checking the brightness against a static 128 value, however it would be nice to be able to adjust this value(I've found, for example, that 170 fits my needs better). Potentially add an option in the constructor, maybe as part of TinyColorOptions: {brightnessThreshold: number}.

I'm aware that you can get around it by doing const isDark = tinyColor.getBrightness() < 170;, but this proposal would allow the user to create a wrapper around TinyColor, and have a default brightness threshold across the entire app, and not have to manually specify it every time.

[scttcper]

sounds very reasonable