feat context: Support resource set default values

This adds functionality to specify default values directly in resource sets.

The idea is that users can create a file called `values.yaml` or `values.json`
in a resource set's folder and have all variables specified in that file be
automatically merged into the resource set variables with the lowest priority.

This fixes #25
This fixes #30 (to a degree)

context/context_test.go

@@ -140,3 +140,17 @@ func TestSubresourceVariableInheritanceOverride(t *testing.T) {
 		t.Fail()
 	}
 }
+
+func TestDefaultValuesLoading(t *testing.T) {
+	ctx, err := LoadContextFromFile("testdata/default-loading.yaml")
+	if err != nil {
+		t.Error(err)
+		t.Fail()
+	}
+
+	rs := ctx.ResourceSets[0]
+	if rs.Values["defaultValues"] != "loaded" {
+		t.Errorf("Default values not loaded from YAML file")
+		t.Fail()
+	}
+}