Allow per-extension configuration in cascade

[kpdecker]

It would be nice to have a config cascade stage that allows for the config to be altered based on file extension and/or glob pattern. The primary use case for this is to allow for linting of jsx and es6 features while still allowing for linting of grunt/gulp/etc files that are required to bootstrap the jsx/es6 source code.

Strawman:

  "extensions": {
    "es6.js": {
      "rules": {
        "no-var": 1
      }
    },
    "jsx": {
      "ecmaFeatures": {
        "jsx": true
      },
      "plugins": [
        "react"
      ]
    }
  },

[nzakas]

There's a bunch of complexities to this, which is why it's not being committed to yet. However, it is part of our discussions for 2.0.0 (#2203), so follow there for updates.