Emacs minor mode to more comfortably read and write long-lined prose

 Olivetti, a minor mode for Emacs, displays content soft-wrapped to a
 comfortable width, centered inside the larger window. The visual effect of
 this is similar to the sheet of paper in WYSIWYG word-processors such as
 LibreOffice, and is similar to distraction-limiting software such as
 FocusWriter, WriteRoom, and various Markdown editors. Olivetti-mode is
 usually used for writing prose and long documents, but when used for reading
 it may slightly increase one's reading speed and one's ability to recall the
 material.
 .
 Features:
   * Exclusively affects buffers for which it has been activated.
   * Dynamically manages window margins so that text remains centered.
   * Interactive modification of `text-body-width` at any time.
   * When `olivetti-body-width` is set to an integer, the text area will
     scale with the size of the font; whereas, when a fraction or float
     is used, the apparent size of the text area will remain visually
     proportioned to the size of the window.
   * Optionally remember the state of visual-line-mode on entry and recall
     its state on exit. Olivetti enables visual-line-mode by default.