Different eslint rules based on file extension

This was added. Example from the docs:

{
  "rules": {
    "quotes": [ 2, "double" ]
  },

  "overrides": [
    {
      "files": [ "bin/*.js", "lib/*.js" ],
      "excludedFiles": "*.test.js",
      "rules": {
        "quotes": [ 2, "single" ]
      }
    }
  ]
}

Effecitively you can supply globs and a set of rules that override the default rules for files that match each glob.