Added a new tool: “Remarkise”

*Remarkise* — render your markdown as remark slides, in real time.

Features:
* When the given URL fails, Remarkise tries a couple possible alternatives (GitHub → raw GitHub → [RawGit](http://rawgit.com/)).
* Rendered slides are linkable (via the [history API](http://www.w3.org/TR/html/browsers.html#apis-for-creating-and-navigating-browsing-contexts-by-name)).
* Very fast to load, as it's a single, small HTML file with no external CSS nor JS.

Demo: visit [`https://tripu.github.io/remark/remarkise`](https://tripu.github.io/remark/remarkise) and enter the URL of a remark file, eg [`https://github.com/w3c/specberus/blob/master/doc/Presentation.md`](https://github.com/w3c/specberus/blob/master/doc/Presentation.md).