Accept functions for highlight options

First attempt at accepting both strings and functions as options for highlights, if workable can easily be extendable to more options(popup, etc.).

Allows for the following options to be functions:
- `highlightOnHover`
- `highlightFillColor`
- `highlightBorderColor`
- `highlightBorderWidth`

Each function will be passed `d` and the `data` following D3.js syntax.

Should potentially also solve issue #24.

@markmarkoh would love your feedback on this POC.