Features:

- Added support for sided mixins in a quilt.mod.json files:
    - See https://github.com/QuiltMC/rfcs/pull/89/files for the spec changes

For example:

"mixin": [
   {
      "config": "modid_client.mixins.json",
      "environment": "client"
   },
   "modid.mixins.json"
]

would load `modid.mixins.json` on both dedicated servers and clients, and not load `modid_client.mixins.json` on the dedicated server.
