Make clean-URL story consistent

[strugee]

Some parts of the system like e.g. stratic-indexes-to-rss create and assume clean URLs without extensions. I should go through all the modules and make sure that they handle URLs consistently.

[strugee]

Here's a list of repositories we need to do this for:

• stratic-handle-offset
• generator-stratic
• stratic-decorate-files
• stratic-date-in-path
• stratic-paginate-indexes
• stratic-indexes-to-rss
• stratic-posts-to-index
• stratic-filter-drafts
• stratic-default-categories
• stratic-truncate-indexes