Add Sass custom theme option. Plus, some clean up.

* Add custom theme support using Sass.
* Simplify API: HTML-slides behave more like markdown-slides, i.e. both can use the `--attributes` option.
* A slide `Object` in `slides/list.json` does not need to have a filename to enable simple image slides.
* Rename `build` task to `dist` task, so it won't be mistaken for `build:index`.
* Remove `coffee-script` as a production dependency. Instead compile `.coffee` by running `npm prepublish`.