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/testdata/parent-variable-override.yaml

@@ -0,0 +1,10 @@
+---
+context: k8s.prod.mydomain.com
+include:
+  - name: parent
+    values:
+      foo: bar
+    include:
+      - name: child
+        values:
+          foo: newvalue

context/testdata/parent-variables.yaml

@@ -0,0 +1,10 @@
+---
+context: k8s.prod.mydomain.com
+include:
+  - name: parent
+    values:
+      foo: bar
+    include:
+      - name: child
+        values:
+          bar: baz