Honor inherited implementations of Chart#transform

Chart#extend and Chart#mixin are intended to help authors create new
chart definitions from existing constructors. These new definitions may
define their own data transformation logic, but they likely expect the
previously-defined transformations to take place as well (which is why
they have chosen to reuse the "base" chart).
Abstract data dereferencing

In order to better facilitate reusability, provide an interface for
working with chart data. With it, chart consumers are able to define the
"shape" of their data. Allow consumers to explicitly opt out of this
behavior for situations that require runtime optimization (i.e. large
datasets, frequently-updating charts, or low-powered devices).
Fix bug in dataAttrs inheritance logic

Account for charts that extend other charts without declaring additional
data attributes.
Do not map non-array-like data

This is a hack to support using mixins for multiple data series. I can't
think of a better solution at the moment
Update examples

They work! But the hybrid chart relies on a hack (see previous commit)
Begin solving data attrs problem?!
Use external library for data mapping