"Color on map" option for blocks and plants

Issue description

Currently, the color a block appears as on a map is based on its material (which means plants will always appear green). With this piece of code, it's possible to change its map color regardless of its material:

@Override
		public MapColor getMapColor(IBlockState state, IBlockAccess worldIn, BlockPos pos) {
			return MapColor.COLOR;
		}

While this is a minor feature, it can help making custom biomes stand out in maps (the attached images show the map of a custom biome before and after applying custom map colors).

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