Adds texture overrides for independent materials

[YANOUSHek]

Allows passing a materialOverrides option to the loadMtl function.
When generating a new material parsed from the .mtl file the overrides
that are specific to this material are applied first before the global
material overrides already existing in the code. This allows the users
to specify different set of materials to override for a model. Following
is an example value for the materialOverrides key:

{
  "material1Name": {
    "alphaTexture": "path_to_the_texture_file",
    "ambientTexture": "path_to_the_texture_file",
    "diffuseTexture": "path_to_the_texture_file",
    "emissiveTexture": "path_to_the_texture_file",
    "normalTexture": "path_to_the_texture_file",
    "specularTexture": "path_to_the_texture_file",
    "specularShininessTexture": "path_to_the_texture_file",
  }
}

It's possible to define more than one material override. If the .mtl
file has materials that were not provided in the materialOverrides
option they are loaded from the .mtl file without any overrides.