add PhantomJS support

I've been working with PhantomJS a fair amount recently and noticed that PhantomJS is not currently one of the supported platforms/environments in JSHint. Considering that it's becoming a fairly popular and widespread tool, I thought this would be useful for the community. This pull request adds that support, and also the test coverage.

It adds `phantom` as the option, which will acknowledge the following symbols:

* `phantom`
* `require`
* `WebPage`


Relevant commit:

7963ba2