Support for setting minimum piechart slice size.

* [x] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md).

## What did you do?

Generate a pie chart with my own dataset, each portion of the pie chart should be easily clickable.

## What did you expect to happen?

Piechart should have a property to set the minimum slice size. E.g if I set that property to 0.1, even though the minimum dataset only contributes to 1% of the whole pie chart, it should show in 0.1.

## What happened instead?

Some sections of the piechart are very hard to click if its portion is small. The following screenshot should explain the issue, there is a really thin slice of pink between yellow and light red. It is almost impossible to click.:

Can anyone review this? If you think it is a reasonable feature, I can work on a PR for it as well. Thanks.