Streamline Custom Series Type Support

[hcpchris]

We need a more streamlined way to support the extension of Highcharts for Python with custom series types, similar to the use of the JavaScript Highcharts.seriesType() method.

Basic pattern to implement:

1. Expect custom series type to be defined as an instance of CustomSeriesType.
2. Expect custom series type to have a class method for series_options which returns a class object for configuring the custom series.
3. Add mechanism to register a custom series type on the chart.
4. Add support for a custom series type to series type validation logic.