Plugins for impress

Here's a suggested structure for plugin support. [Deck.js](https://github.com/imakewebthings/deck.js) and [Reveal.js](https://github.com/hakimel/reveal.js) both have plugin support and get a lot of good functionality in through plugins.

This feature introduces a dependency on a custom build of Modernizr (6.107 kb) supporting script loading and touch event detection. This might get heavier in the future as new plugins require new features but it's probably not the end of the world. Resource loading will be very practical for a lot of developers allowing them to load additional resources required by their plugins asynchronously.   

Example plugin (disabled by default) a suggested structure of the plugin and shows resource loading and interaction with the impress instance.