Combo charts

First of all, thanks for a awesome library! I use it with your ruby gem and it's much easier then any other charting library i have ever used!

I needed a combination of line and column chart in one of my projects and found that both google and highchart include support for this, without loading any other libraries so i decided to implement support for this into chartkick, so here goes :)

The types is added as an array very similiar to how you set colors:
```javascript
new Chartkick.ComboChart("chart-1", [["Work", 32],["Play", 1492]], types: ["line", "column"]);
```
If you think their is a better way to implement this option, will i gladly change and improve it.

#### Examples:
Highchart:
http://www.highcharts.com/demo/combo

Google Charts:
https://google-developers.appspot.com/chart/interactive/docs/gallery/combochart