Add module for slim-mode, and leave it disabled for now

After experimenting with slim for a project, I decided against
it. However I'd created a module for it for my emacs config here, so
might as well keep it around for next time I come across slim.

core/siren-modules.el

@@ -101,6 +101,7 @@
 (require 'siren-thrift)
 (require 'siren-web-mode)
 (require 'siren-yaml)
+;; (require 'siren-slim)
 
 (provide 'siren-modules)
 ;;; siren-modules.el ends here