Add `builder` method to create a new instance of the builder

[josejulio]

When using (in the context of the maven builder) useInnerClassBuilders and generateBuilders set to true it would be nice to have a builder method to generate a new builder.

i.e.
Instead of:

new MyGeneratedObject.MyGeneratedObjectBuilder()
    .withFoo("bar")
    .build();

Have something like:

MyGeneratedObject.builder()
    .withFoo("bar")
    .build();

It could be an opt-in by a new setting e.g. addBuilderMethodForInnerClassBuilders.

I'm supposing the same applies for the core, but haven't checked all the details other thant the BuilderRule file where I suppose it needs to be modified.

[joelittlejohn]

No need for a setting for this. We can add the method in a way that does not break existing code, and I agree it would be nice to include it.