Human readable SLP adding

[abductedPlatypus]

A config file like:

{
    "slp_replace": {
        "MED_WATER"  : "lava.slp", //replace with file
        "GRASS2": "GRASS3", // replace with default texture
        "DIRT3": "DIRT4" // DIRT4 is an HD texture, but we will allow it here
    },
    "use_grid_textures": true'
}