Add support for FreeMarker in non-webapp

The existing freemarker support only works in a webapp. This
change adds a FreeMarker Configuration bean (in both web- and
non webapps) so it can be used to load a Template and render it
(e.g. with Spring's FreeMarkerTemplateUtils).

See gh-679

spring-boot-autoconfigure/src/test/resources/templates/message.ftl

@@ -0,0 +1 @@
+Message: ${greeting}