Create a `palette-overrides` feature similar to the current `palette-primers` feature

[grunt-lucas]

• palette-overrides folder in the input folder
  • in this folder, numbered JASC PAL files containing exactly 16 colors are copied directly into the final palette
    • e.g. 1.pal will become palette 1
    • fail build if we can't assign all tiles given the override
    • user can put a - on a line to indicate that this could be any color
    • the primary use case for this would be so that users could:
      • 1. e.g. fix a given color slot in a given palette, sometimes this is nice for certain DNS systems, etc
      • 2. manually construct their palettes if they prefer, but still benefit from Porytiles automatic metatile generation and tiles.png compression