Title-Footer plugin

This plugin includes a footer in all the slides (with optional exclusion of some slides) showing the title of the presentation. It takes the first ```h1```, ```h2``` and ```h3``` tags from the first slide as the title, if another title is not explicitly passed as parameter in the initialization. The title footer will not be shown in a slide if the corresponding section has a ```data-state``` attribute with a value of ```no-title-footer```. You can find an example of it working at [http://e-gor.github.io/Reveal.js-Title-Footer/demo](http://e-gor.github.io/Reveal.js-Title-Footer/demo) and instructions of use at [https://github.com/e-gor/Reveal.js-Title-Footer](https://github.com/e-gor/Reveal.js-Title-Footer).

Example configuration:

```javascript
Reveal.initialize
(
	{
		...
		dependencies:
		[
			...
			{ src: 'plugin/title-footer/title-footer.js', async: true, callback: function() { title_footer.initialize(); } }
		]
	}
);
```

The ```title_footer.initialize``` function can take two parameters:

- ```title```: the title to show in the footer; if ```null```, it will take the ```h1```, ```h2``` and ```h3``` elements of the first slide.
- ```background```: a string of the form ```'rgba(0,255,0,0.1)'```, for the background colour of the footer.