Demos: Generate Working examples from meta data

Proof of concept for generating the working examples page off of Assemble page collections. 

All the examples would need to move some of the data to the additional front matter:
- Description: Usually the first sentence of the demo, but it would need to fit on the table
- Category: Not sure if this should be the Plugin/Polyfill name or if it is a plugin or polyfill
- Keywords/Tags: for additional search/filtering params

TODO:
- [x] Differentiating between different demos like with mediaplayer
- [x] French/i18n version
- [x] Additional metadata for all examples
- [x] Excluding any AJAX templates
- [ ] Sidebars like http://assemble.io/docs/?