feat templater: Add 'pass' lookup function

This introduces support for looking up secret values in the 'pass' command line
tool (https://www.passwordstore.org/).

Values like passwords can be interpolated from pass and even more complex
structures like certificates for Kubernetes Secrets can be retrieved and base64-
encoded as necessary.

Fixes #2

templater/templater.go

@@ -158,6 +158,7 @@ func templateFuncs() template.FuncMap {
 		b, _ := json.Marshal(data)
 		return string(b)
 	}
+	m["passLookup"] = GetFromPass
 
 	return m
 }