Supporting nested description

I suggest to use "closest describe".
Because I want to nested description 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
```