Allow for a custom end color during color interpolation.

[brian-mcallister-lab49]

Right now, you can pick custom colors (bidColor and askColor) which will get interpolated over the length of the order book. But, those colors simply get interpolated into white. Instead, we can add props to let users choose a custom end color. This could be quite useful when trying to match the visual design of your order book to an existing application's visual style.