Create as `grunt-init` project template

## About

It make sense to offer reveal.js as a `grunt-init` [project template](http://gruntjs.com/project-scaffolding).

This allows users to just to create their new presentation with the following command.

    grunt-init reveal


## Pull request

I've done the necessary to create the `grunt-init reveal` task. However, I'm not sure how this code should be offered.

Should it be part of this repo (the most obvious), maybe as a separate branch?
Or is it better to keep it into a separate repo (another one to maintain)?

Any thoughts on this?