To add arbitrarily formatted string of {value}.

Hi Ben,

Thanks for the library. 

Currently, there is no easy way to add formatted value, i.e `1000.123 -> $1,000.12`, except for using `{caption}`. However, that means that I may not be able to display the percentages in the tooltip. `{f_value}` basically accepts any arbitrary values/strings for now. 
Hence, instead of:
```javascript
"string": "{label}: {value}, {percentage}%"
```
one can use:
```javascript
"string": "{label}: {f_value}, {percentage}%"
```

Further consideration can be put in place to make use of [Numeral-js](https://github.com/adamwdraper/Numeral-js). Currently, I am using both libraries independently:
```javascript
datapoint.f_value = numeral(value).format('$0,0.00');
```
The concept is similar to what Google Charts API has on displaying a formatted value, i.e. ArrowFormat example in [Google Docs API - Formatters](https://developers.google.com/chart/interactive/docs/reference#formatters).

Thanks once again.

Robert