lang: funcs: Add datetime.format function

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

examples/lang/datetime4.mcl

@@ -0,0 +1,10 @@
+import "datetime"
+import "sys"
+import "example"
+
+$now = datetime.now()
+
+file "/tmp/mgmt-datetime" {
+	state => "exists",
+	content => template("Il est l'or Monseignor: {{ . }}\n", datetime.format($now, "15:04:05")),
+}