Extract JS from HTML files

This is a feature proposal. I'm well aware that inlining JavaScript into HTML files is a bad practice in so many cases, but there is a few defendable usages.

At my work, we develop widgets mainly self-contained into a single HTML file (that's what we did since day one and it works great). In order to lint the JS contained into those files, I had to fork node-jslint in order to extract the source code from the HTML. As we now want to migrate to Jshint, I did the same, and now proposing my changes to the community, as I am sure that it can be useful to someone else.

As this is my first work on jshint, feel free to comment my code and ask me to reshape it if needed.

Edit: I didn't see the ticket #510 before pull requesting, I'm sorry. But I stay on my position that this feature would be better integrated to jshint cli, in order to be compatible with all IDE / text editor jshint plugins. This is also why I don't want to have to pipe the output of a command to jshint in order to validate HTML files.