feat(templater): Add insertTemplate function

Similar to insertFile, but runs the templating against the file before
inserting.

This is useful for sharing common config between yaml files, e.g. volume
mounts in a deployment.yaml and cronjob.yaml, and it's useful to be able
to use the `configHash` annotation pattern with a templated
configmap.yaml

docs/templates.md

@@ -72,13 +72,15 @@ right.
 
 Some template functions come from Go's standard library and are listed in the
 [Go documentation][]. In addition the functions declared by [sprig][] are
-available in kontemplate, as well as three custom functions:
+available in kontemplate, as well as five custom functions:
 
 * `json`: Encodes any supplied data structure as JSON.
 * `gitHEAD`: Retrieves the commit hash at Git `HEAD`.
 * `passLookup`: Looks up the supplied key in [pass][].
 * `insertFile`: Insert the contents of the given file in the resource
   set folder as a string.
+* `insertTemplate`: Insert the contents of the given template in the resource
+  set folder as a string.
 
 ## Examples:
 

templater/templater.go

@@ -205,6 +205,14 @@ func templateFuncs(c *context.Context, rs *context.ResourceSet) template.FuncMap
 
 		return string(data), nil
 	}
+	m["insertTemplate"] = func(file string) (string, error) {
+		data, err := templateFile(c, rs, path.Join(rs.Path, file))
+		if err != nil {
+			return "", err
+		}
+
+		return data.Rendered, nil
+	}
 	m["default"] = func(defaultVal interface{}, varName string) interface{} {
 		if val, ok := rs.Values[varName]; ok {
 			return val