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/testdata/default-loading.yaml

@@ -0,0 +1,6 @@
+---
+context: default-loading
+include:
+  - name: default
+    values:
+      override: notAtAll