Genfu Instances

[marcoregueira]

It would be great to be able to create GenFu instances. That would reduce interferences in multithreaded scenarios, allow creating specialized generators and better support for DI environments.

For instance:

var childGenerator = new GenFuInstance();
var adultGenerator = new GenFuInstance();

childGenerator.Configure<Person>().Fill(x => x.Age).WithinRange(1, 10);
adultGenerator.Configure<Person>().Fill(x => x.Age).WithinRange(20, 100);

PR #146 implements this change with while keeping old static usage to avoid breaking changes.