Support custom formatting of chart labels

[franqueli]

We have a use case where we need some custom logic when displaying a label on a bar chart. A zero value would at times need to be labeled N/A. I don't think the current interface makes it easy to support this.

I've implemented this functionality modeled after MPAndroidChart's ValueFormatter interface.
Changes can be viewed in this git repo.
https://github.com/franqueli/ios-charts/tree/ValueFormatterProtocol

[danielgindi]

Implemented in Charts 3.0 (v3 branch)