Normalize slug file name

!!! Breaking change !!!

If you have been using slug, you may have realized that filenames get normalized skipping spaces. However when there was something like test -- something it would get normalized to test----something. Instead, now any time there is a situation like this, it'll get normalized to one dash. In the above case, it'd be test-something.

Also, fixed a minor bug: on the preview, the draft indicator now works properly.