feat(templater): Added support for file include

Adds a 'fileContent' template function to insert the literal contents
of a file specified in the template.

Signed-off-by: Niklas Wik <niklas.wik@nokia.com>

example/some-api/some-api.yaml

@@ -25,6 +25,8 @@ spec:
               value: {{ .importantFeature }}
             - name: SOME_GLOBAL_VAR
               value: {{ .globalVar }}
+            - name: FILE_VAR
+              value: {{ fileContent "some-api/filevar.txt" }}
 ---
 apiVersion: v1
 kind: Service