Consider allowing bindingContext to be a function that gets called after aurelia.start()

[silbinarywolf]

Why
Consider allowing bindingContext to be a function as well.
A potential benefit is that you can use "Container.instance.get" after aurelia has called start.

const bindingContext = this.bindingContext;
//const bindingContext = typeof(this.bindingContext) === 'function' ? this.bindingContext() : this.bindingContext;