Make external hash checker an option

Some websites, like [Travis](http://docs.travis-ci.com/user/build-configuration/#White--or-blacklisting-branches) and <https://enterprise.github.com/faq>, use JavaScript to generate header links for the TOC. This screws up the newly introduced external hash checker, so let's move that into an option. 

You can always explicitly ignore an href with `href_ignore`, but you shouldn't have to create an ignore if you don't want to.