Supporting nested describe

Hi,

I suggest to use closest describe.
Because I want to nested describe as below.

* lib/tasks/foo.rake

```
namespace :foo do
  task :bar => :environment do
  end

  task :baz => :environment do
  end
end
```

* spec/tasks/foo_spec.rb

```
describe "foo" do
  include_context "rake"

  describe "foo:bar" do
    it { subject.invoke }
  end

  describe "foo:baz" do
    it { subject.invoke }
  end
end
```