region check - Shadow DOM support

[marcysutton]

Failing tests for the region check:

    14) should return true when wrapper content is outside the region
    ✓ should return true when invisible content is outside the region
    ✓ should return true when there is a skiplink
    15) should return false when there is a non-region element
    ✓ should return the first item when after is called
    16) should return false when there is a non-skiplink

Subtask of #87.

Full stack trace:

14) region should return true when wrapper content is outside the region:
     
  visible@http://localhost:9876/axe.js:9427:31
  checkRegion@http://localhost:9876/tmp/rules.js:1749:72
  map@[native code]
  getViolatingChildren@http://localhost:9876/tmp/rules.js:1759:24
  evaluate@http://localhost:9876/tmp/rules.js:1766:32
  http://localhost:9876/test/checks/navigation/region.js:47:44
  callFn@http://localhost:9876/node_modules/mocha/mocha.js:4447:25
  run@http://localhost:9876/node_modules/mocha/mocha.js:4439:13
  runTest@http://localhost:9876/node_modules/mocha/mocha.js:4936:13
  http://localhost:9876/node_modules/mocha/mocha.js:5042:19
  next@http://localhost:9876/node_modules/mocha/mocha.js:4853:16
  http://localhost:9876/node_modules/mocha/mocha.js:4863:11
  next@http://localhost:9876/node_modules/mocha/mocha.js:4787:16
  http://localhost:9876/node_modules/mocha/mocha.js:4831:9
  timeslice@http://localhost:9876/node_modules/mocha/mocha.js:82:27

  15) region should return false when there is a non-region element:
     expected true to be false
  

  16) region should return false when there is a non-skiplink:
     expected true to be false