Create chart block extension

Rough draft using chart.js
Ref #2

**Given**
sample.adoc
```
= Chart Block Macro Extension

chart::line[data-uri="sample.csv"]
```
sample.csv
```csv
January,February,March,April,May,June,July
28,48,40,19,86,27,90
65,59,80,81,56,55,40
```

**Then**
![chart-ext](https://cloud.githubusercontent.com/assets/333276/3787635/9ec320fa-1a28-11e4-9ac1-4ff48d057b48.png)