Add support for running access transformers on remapped artifacts

FML access transformers change private/protected/public or
final access to classes, methods, or fields. Used in FML and Forge
to grant mods access to Minecraft internals for modding purposes.

Example usage:
<accessTransformers>
    <param>https://raw.github.com/MinecraftForge/FML/20337b6e8aec12d2793edb376bdf8dea119cba5f/common/fml_at.cfg</param>
    <param>https://raw.github.com/MinecraftForge/MinecraftForge/4299aa22ffcc7770499b5edc4ec330da042a5eb8/common/forge_at.cfg</param>
</accessTransformers>