Add support for 'extends'

[lamchau]

The extends on JSHint feature has been supported for awhile but during a recent migration discovered this isn't supported.

Not entirely sure how this should be resolved, would broccoli-jshint need to manually merge .jshintrc files?

Steps to repro.

// app/.jshintrc
{
  node: true
}

// app/tests/.jshintrc
{
  "extends": "../.jshintrc"
}

Livereload server on port 35729
Serving on http://localhost:4200/
self is not defined
ReferenceError: self is not defined
    at JSHinter.getConfig (C:\dev\project\node_modules\ember-cli-qunit\node_modules\broccoli-jshint\index.js:114:7)
    at C:\dev\project\node_modules\ember-cli-qunit\node_modules\broccoli-jshint\index.js:37:28
    at lib$rsvp$$internal$$tryCatch (C:\dev\project\node_modules\ember-cli\node_modules\rsvp\dist\rsvp.js:489:16)
    at lib$rsvp$$internal$$invokeCallback (C:\dev\project\node_modules\ember-cli\node_modules\rsvp\dist\rsvp.js:501:17)
    at lib$rsvp$$internal$$publish (C:\dev\project\node_modules\ember-cli\node_modules\rsvp\dist\rsvp.js:472:11)
    at lib$rsvp$asap$$flush (C:\dev\project\node_modules\ember-cli\node_modules\rsvp\dist\rsvp.js:1290:9)
    at process._tickCallback (node.js:355:11)

// output
unit/components/my-component-test.js: line 0, col 0, Bad option: 'extends'.