initial helper for optional host_directory

Creates a helper to display a default text value when the provided value is not present. The default value will be styled different than a provided one. I thought about having the helper create the whole host : container line but this might be more useful as a generic optional value helper.

Interesting, I've never used haml_tag before. Is there a strong preference for this over content_tag? content_tag will work in haml templates, but I assume haml_tag would not work in erb templates, so perhaps content_tag would be the safer option.