add unbuild to @builder

[dabraham02124]

In addition to all the neat stuff in the @builder annotation, could you please add an unbuild method? This would create a builder based on the object in question.
Thus if object personName1 has firstName = "John" and lastName = "Smith" you could do this:

personName2 = personName1.unbuild().lastName("Johnson").build();

yielding personName2 with firstName = "John" and lastName = "Johnson"

Also you could implement clone() pretty directly from this if you wanted.

[benmccann]

+1 (though I'd prefer that the method be named builder())

[dabraham02124]

That makes sense. I'd been reasoning by contrast with build(), but this method would be on the actual object, not the builder, so that makes sense to me.