feat ctx: Let sub-resource-sets inherit vars from parent

Users of kontemplate may expect variables defined on the parent resource to be
inherited by children.

This implements that functionality. Values defined twice are overwritten by the
child's definition.

Fixes #20

context/context_test.go

@@ -81,3 +81,62 @@ func TestLoadContextWithResourceSetCollections(t *testing.T) {
 	}
 
 }
+
+func TestSubresourceVariableInheritance(t *testing.T) {
+	ctx, err := LoadContextFromFile("testdata/parent-variables.yaml")
+
+	if err != nil {
+		t.Error(err)
+		t.Fail()
+	}
+
+	expected := Context{
+		Name: "k8s.prod.mydomain.com",
+		ResourceSets: []ResourceSet{
+			{
+				Name: "parent/child",
+				Values: map[string]interface{}{
+					"foo": "bar",
+					"bar": "baz",
+				},
+				Include: nil,
+				Parent: "parent",
+			},
+		},
+		BaseDir: "testdata",
+	}
+
+	if !reflect.DeepEqual(*ctx, expected) {
+		t.Error("Loaded and expected context did not match")
+		t.Fail()
+	}
+}
+
+func TestSubresourceVariableInheritanceOverride(t *testing.T) {
+	ctx, err := LoadContextFromFile("testdata/parent-variable-override.yaml")
+
+	if err != nil {
+		t.Error(err)
+		t.Fail()
+	}
+
+	expected := Context{
+		Name: "k8s.prod.mydomain.com",
+		ResourceSets: []ResourceSet{
+			{
+				Name: "parent/child",
+				Values: map[string]interface{}{
+					"foo": "newvalue",
+				},
+				Include: nil,
+				Parent: "parent",
+			},
+		},
+		BaseDir: "testdata",
+	}
+
+	if !reflect.DeepEqual(*ctx, expected) {
+		t.Error("Loaded and expected context did not match")
+		t.Fail()
+	}
+}