[FR] Add option to load palettes from file system

[randyrossi]

[PaulSlocum]

I'm adding my own request for this feature. I need this because although the colors come out great on a CRT, some colors (especially green) don't look right when using HDMI with an LCD monitor. On every LCD monitor, green looks more saturated than it does on any of my NTSC CRTs (I only have NTSC gear). I'm unable to suitably fix the issue with any of the built-in palettes or by changing video color settings.