feat(templater): Support single-template resource sets

Supports resource sets in which the `path` is pointed at a single
template file.

The example has been updated with ... an example of this.

This closes #81.

example/other-config.yaml

@@ -0,0 +1,7 @@
+---
+apiVersion: extensions/v1beta1
+kind: ConfigMap
+metadata:
+  name: other-config
+data:
+  globalData: {{ .globalVar }}

example/prod-cluster.yaml

@@ -3,8 +3,15 @@ context: k8s.prod.mydomain.com
 global:
   globalVar: lizards
 include:
+  # By default resource sets are included from a folder with the same
+  # name as the resource set's name
   - name: some-api
     values:
       version: 1.0-0e6884d
       importantFeature: true
       apiPort: 4567
+
+  # Paths can also be specified manually (and point at single template
+  # files!)
+  - name: other-config
+    path: other-config.yaml