Companion objects for API classes

[fmcgough]

It would be really useful if the generated API classes such as HighchartsConfig had companion objects, with an apply() method to construct instances of those classes, using default values of js.undefined. Example usage could look something like this:

val config = HighchartsConfig(title = Title(text = "My Cool Chart", x = -20), ...)

It would save a lot of boilerplate and make the library feel more natural in a Scala environment.

Thanks :)