Add word_wrap option to settings

[DannyBen]

cc #687

Changes:

• String#wrap no longer has a default value of 80 (it was never used by anyone)
• Settings now has a new word_wrap option, defaults to 80
• Renderable concern now has a new user_string helper that allows this diff with the same result:

  - printf "{{ help.wrap(76).indent(4).sanitize_for_print }}\n"
  + printf "{{ user_string help, indent: 4 }}\n"
  

  Word wrap is calculated by using the settings value minus indent (same behavior, only with user value).