Add ability to conditional run some tests

[matryer]

As a developer I want to omit long running tests So that I can quickly test new code without having to run every test function in a file.

---

Perhaps I could do:

gort --without=threads

Then in code, maybe this:

func TestThreads(t *testing.T) {
  gort.Group("threads", func(t *testing.T){
     // test code goes here
  })
}