Avenue

Besides some small improvements (_, like getting rid of the bottom-borders on headings, better tables, and more_), main reason for this pull request is allowing the user to decide the rendered width of the text _in preferences_; before there was this set `max-width: 30em` on the `<p>`-tag in the stylesheet, which feels redundant in the preview-app environment, because of this preference-option :+1:  (_which I somehow wasn't aware of before :)_