Add focus for describe blocks

[jqr]

Allow calling focus on/before a describe block to focus on all tests within.

describe "My Test" do
  focus
  describe "Group 1" do
    it "will run this" {}  # previous was _just_ this focus
    it "and this"      {}  # now runs this as well
    describe do
      it "this too"    {}  # double nesting also
    end
  end
end