Sanitize of Tooltip breaks the ability to style a tootlip or return multiple values

Adding the sanitize (issue #1536) has taken away the ability to style or do more complex things in the tooltip.

I was using the format value to have both the value and the ratio returned for a pie chart and styled it by returning a html string

``` javascript
tooltip: {
format: {
value: function(value, ratio) {
var percentFormat = d3.format('.1%');
var twoDecimal = d3.format('.2f');
return '

' +
'
• ' + percentFormat(ratio) + ' ' +
  '
• ' + twoDecimal(value) + '
' +
'
';
}
}
}
```

now with the sanitize this breaks I am unable return multiple values returned or stylized tooltip values.

maybe something like node plugin [sanitize-html](https://github.com/punkave/sanitize-html) and whitelist some tags instead of just putting it as a string.