Add support for graphing libraries such as charts.js / highcharts.js

It would be pretty neat if we could render charts & graphs live inside slides by making use of graphing libraries such as charts.js or highcharts.

Ideally, we should one need to define a keyword + link to a CSV to initiate rendering. Maybe remark.js macro functionality would be a good fit for this. It could look like this:

```

---
title: Graph
graph: /path/to/data/source.csv

---
```
