Ability to specify formatting in tooltip

Support for text transformation via [vega marks template](https://github.com/vega/vega/wiki/Marks#templates) or just simply [d3 formatting](https://github.com/d3/d3-3.x-api-reference/blob/master/Formatting.md) for tooltip, labels, ticks in `definition` like

```
"tooltip": {
"title": "{aspect}",
"content": "{totalPerAspect|number:'$.3f'}"
}
```
or maybe in `override`

```
"marks": [
{"properties":
{
text": {"template": "{totalPerAspect|number:'$.3f'}"},
}
}
]
```