Add player_textures-like functionality for custom skins.

As in the title, I just spent a bit of time and put in the player_textures functionality for custom skins. You drop a "player_playername.png" in nc_player_model's textures folder and it will set the player's base texture to that instead of the normal "nc_player_model_base.png" The copied part of the code is WTFPL.