feat context: Add ability to import extra variables from files

Kontemplate context specifications can now load extra variables from YAML
or JSON files by specifying a list of files (relative to the context file)
under the `import` key.

context/testdata/import-vars-override.yaml

@@ -0,0 +1,9 @@
+---
+context: k8s.prod.mydomain.com
+global:
+  globalVar: very global!
+  override: 1
+import:
+  - test-vars.yaml
+  - test-vars-override.yaml
+include: []