Implement link validation

[Tyriar]

Fixes #570

---

This allows consumers to invalidate links:

term.registerLinkMatcher(/\w+\.json/, () => console.log(Date.now()), {
  validationCallback: function (uri, cb) {
    // Only enable URIs that exist on disk
    cb(fs.existsSync(uri));
  }
});

As part of this I also removed PhantomJS in favor of much more lightweight jsdom. This makes the tests far more readable, is way faster to download and run and everything runs through the mocha gulp task so they're included in the coverage report!

The one downside with jsdom is that it doesn't do layouts so we have to fake them in the tests, this is a small price to pay for the benefits though imo.

[coveralls]

Coverage decreased (-0.2%) to 64.43% when pulling d0f36e9 on Tyriar:570_link_validation into cf4da6c on sourcelair:master.

[coveralls]

Coverage increased (+2.7%) to 67.358% when pulling b401992 on Tyriar:570_link_validation into cf4da6c on sourcelair:master.

[coveralls]

Coverage increased (+2.9%) to 67.529% when pulling 356d86a on Tyriar:570_link_validation into 3e5fcf7 on sourcelair:master.

[coveralls]

Coverage increased (+2.9%) to 67.529% when pulling 3542f15 on Tyriar:570_link_validation into 3e5fcf7 on sourcelair:master.

[coveralls]

Coverage increased (+2.8%) to 67.468% when pulling 3542f15 on Tyriar:570_link_validation into 3e5fcf7 on sourcelair:master.

[parisk]

Looks great. I am also fond of ditching Phantom in favor of jsdom.

Just a couple of comments on file naming.

[parisk]

Should we change the naming from *.phantom.js to *.jsdom.js?

[Tyriar]

Opps, the test harness was meant to be deleted. Fixed in next commit. jsdom tests are now run with the regular tests so no need for a custom name.

[parisk]

Same here, should we change the naming from *.phantom.js to *.jsdom.js since we ditched PhantomJS in favor of jsdom?

[coveralls]

Coverage increased (+2.8%) to 67.468% when pulling 6990653 on Tyriar:570_link_validation into 3e5fcf7 on sourcelair:master.