CLI: when the user sets the title, create the offline HTML filename from the slugified title

[joanise]

In the Web App, the Office-HTML filename is based on the slugified title.

We don't want to do that when the user didn't set the title (i.e., when the title is the default string), but when the user provides a title it would be a good idea.

Suggested by @deltork in review comments to #231