Doughnut scriptable options

[etimberg]

Background

This PR introduces scriptable configuration options for doughnut charts. The user can script the styling, interaction, and border alignment options.

Testing

Unit tests were written to cover these features and were based on the existing bubble controller tests. The existing unit tests covering the setHoverStyle dataset controller method were removed as they did not have proper setup for use with scriptable options.

Documentation

I updated the doughnut chart documentation to match the style of the bubble & bar charts as they already support scriptable options.

[nagix]

Looks good to me except for a few minor errors. Are you going to add fixture tests and a sample?

[etimberg]

I will add fixture tests but I don't have anytime today so I can't get to it until tomorrow. I'll see what we have for the bar & bubble charts in terms of this. The interaction tests should cover a fair bit of it already.

[simonbrunel]

Looks good, I agree with @nagix that we should also add image based unit test, similar to these ones.

[etimberg]

@simonbrunel @nagix I added some scriptable fixture tests for backgroundColor, borderColor, and borderWidth. I probably should write one test for borderAlign and then this should be good to go

[etimberg]

I've added tests for borderAlign so this should be good to go.

Addressed concerns regarding missing tests

[kurkle]

Looks very good, except for the duplication of couple of lines, reported by codeclimate.
We'll see how it turns out when more scriptable options are implemented and refactor later if needed.