Initial implementation of JSLint gulp plugin

@milang The opt-in setting isn't very discoverable for, say, a new developer who is setting up their development environment. The feature seems useful, but I don't think people will know about it unless it's documented somewhere. Perhaps in the `README` of this project, or under the "How to build" section of the projects that are eventually going to use this plugin ([web.core](https://github.com/pandell/web.core), [pli.core](https://github.com/pandell/pli.core), etc?).